lam pt status of capture fisheries activities and mangemen

STATUS OF CAPTURE FISHERIES ACTIVITIES AND

MANAGEMENT IN TRI AN RESERVOIR, VIETNAM

by

Phan Thanh Lam

A thesis submitted in partial fulfilment of the requirements for the Master

of Science Degree in International Fisheries Management

Department of Economics and Management

Norwegian College of Fishery Science

University of TromsØ, Norway

May 2006

MSc. Thesis

STATUS OF CAPTURE FISHERIES ACTIVITIES AND

MANAGEMENT IN TRI AN RESERVOIR, VIETNAM

by

Phan Thanh Lam

A thesis submitted in partial fulfilment of the requirements for the Master of

Science Degree in International Fisheries Management

Department of Economics and Management

Norwegian College of Fishery Science

University of TromsØ, Norway

May 2006

- i -

ACKNOWLEDGEMENTS First and foremost, I would like to express sincere gratitude to my supervisor Ass.

Prof. Arne Eide for his full support and supervision both during the preparation of

the proposal and the write up of the thesis. Then I would like to thank all the

Professors and Coordinators that have contributed to making these two years here at

the Norwegian College of Fishery Science very memorable and educational.

My sincere gratitude goes to the Norwegian government, the State Education Loan

Fund for granting me the scholarship to undertake graduate studies at the University

of Tromsø. I would also like to thank the SEMUT for its financial support that

enabled me to undertake my fieldwork. At large, thanks to NFH for all the quality and

effort you put in for the running of this International Master course.

The data on which this paper is based were collected under the Dong Nai Fisheries

Company, Vietnam. Special gratitude goes to the Dong Nai Fisheries Company

officials for their permission to carry out the study. I am highly grateful to Mr. Dau

Trong Bang, Vice Director, and Mr. Phan Trung Liet, Chief of the Technical Fisheries

Division of the Dong Nai Fisheries Company, for their support and cooperation during

my fieldwork. I also thank to Dr. Nguyen Thanh Tung and my colleagues at Research

Institute for Aquaculture No.2, for their invaluable support and suggestions throughout

the course of this study. I express thank to Dr. Nguyen Thanh Hung and Ms. Paula

Brown for their comments and English review of manuscript.

I dearly thank my family, my wife, little son and mother-in-law, for their never-ending

love and moral support. Finally yet importantly, I thank all my IFM classmates, UiTØ

and Vietnamese friends who have been part of my stay in Tromsø, it was a pleasure

and experience to have acquainted with you all.

Tromsø, 12 May 2006

Phan Thanh Lam

- ii -

TABLE OF CONTENTS

ACKNOWLEDGEMENTS ................................................................................................i

LIST OF FIGURES........................................................................................................... iii

LIST OF TABLES.............................................................................................................iv

ABBREVIATIONS............................................................................................................v

ABSTRACT.......................................................................................................................vi

I. INTRODUCTION.........................................................................................................2

1.1. Introduction ......................................................................................................2

1.2. Main concerns of the study..............................................................................3

II. BACKGROUND OF FISHERY IN TRI AN RESERVOIR ......................................5

2.1. Characteristics of fisheries resources ..............................................................5

2.2. Status of fish stocking program.......................................................................9

2.3. Status of capture fisheries activities ..............................................................11

2.4. Fisheries management practices ....................................................................14

III. MODEL ......................................................................................................................17

3.1. The logistic growth model .............................................................................17

3.2. Modified surplus production model ..............................................................18

3.3. Biological analysis .........................................................................................22

3.4. Economic analysis .........................................................................................23

IV. DATA AND PARAMETERS ESTIMATION .......................................................27

4.1. Type of data ...................................................................................................27

4.2. Data analysis ..................................................................................................29

4.3. Parameters estimation ....................................................................................34

V. RESULTS ....................................................................................................................35

5.1. Trends in catch, effort, CPUE and stocking..................................................35

5.2. Impact of stocking on population growth and harvest regime .....................38

5.3. Estimations of reference points and economics rents ...................................39

VI. DISCUSSION AND CONCLUSION.......................................................................44

6.1. Discussion ......................................................................................................44

6.2. Conclusion and management implications....................................................51

REFERENCES .................................................................................................................57

APPENDICES ..................................................................................................................62

- iii -

LIST OF FIGURES

Figure 2.1. Map of Tri An reservoir showing the location and study area..................5

Figure 2.2. Photographs of main stocked fish in Tri An reservoir ..............................7

Figure 2.3. The distribution of stocked species from 1995 to 2003 ............................9

Figure 2.4. Trends of fish stocking program from 1993 to 2005 ..............................10

Figure 2.5. Trend of catch and effort from 1993 to 2005 ..........................................13

Figure 3.1. Population growth according to the logistic curves ................................20

Figure 3.2. Plot of population growth versus stock biomass and stocking rate .............20

Figure 3.3. Plot of harvest curve versus effort level and stocking rate ......................21

Figure 3.4. Plot of profit curve versus effort level and stocking rate ........................24

Figure 3.5. Plotted curves of TR, TC vs. effort and stocking....................................26

Figure 5.1. Trends of actual catch, effort and CPUE over time ................................35

Figure 5.2. Trends in standardized catch, effort and CPUE, 1999-2005...................36

Figure 5.3. Trends of actual catch and standardized catch, 1999-2005.....................37

Figure 5.4. Plotted curve of standardized CPUE vs. effort and stocking ..................38

Figure 5.5. Population growth curve vs. stocking and stock biomass levels.............38

Figure 5.6. Plotted curve of harvest vs. effort and stocking ......................................39

Figure 5.7. Plotted curve of profit vs. effort and stocking.........................................40

Figure 5.8. Harvest curve vs. stocking and effort at harvest condition 3 ..................43

Figure 5.9. Profit curve vs. stocking and effort at harvest condition 3......................43

- iv -

LIST OF TABLES

Table 2.1. Estimating the maximum stocking density for Tri An reservoir ..............11

Table 2.2. Major fishing gears used in Tri An reservoir ...........................................12

Table 4.1. Actual catch and efforts data of capture fisheries, 1993-2005 .................27

Table 4.2. Average investment fishing costs of households (2004-2005).................28

Table 4.3. Fish stocking activity in Tri An reservoir, 1993-2005 .............................29

Table 4.4. Effort standardizations by type of fishing gears from 1999 to 2005 ........31

Table 4.5. Cost-efficiency analysis by fishing gears .................................................33

Table 4.6. Estimated parameters based on non-linear regression model...................34

Table 5.1. Standardized catch, effort and CPUE, 1999-2005....................................36

Table 5.2. Calculation indicators of the reference points and economic rents ..........41

- v -

ABBREVIATIONS

AMCF Assessment of Mekong Capture Fisheries

CPUE(E, S) Catch per Unit of Effort as a function of Effort and Stocking

DNFC The Dong Nai Fisheries Company

E Effort

EMEY Effort at Maximum Economic Yield

EMSY Effort at Maximum Sustainable Yield

EOAY Effort at Open Access Yield

FAU The University of Agriculture and Forestry

H(E, S) Harvest as a function of Effort and Stocking

MEY Maximum Economic Yield

MoF Ministry of Fisheries

MRC Mekong River Commission

MSY Maximum Sustainable Yield

MSPM Modified Surplus Production Model

OAY Open Access Yield

R2 The coefficient of determination

RIA2 Research Institute for Aquaculture No.2

rs Intrinsic growth rate affected by fish stocking

S Stocking

SMEY Stocking at Maximum Economic Yield

SMSY Stocking at Maximum Sustainable Yield

SOAY Stocking at Open Access Yield

SPM Surplus Production Model

TC(E, S) Total Cost as a function of Effort and Stocking

TR(E, S) Total Revenue as a function of Effort and Stocking

VNDs Vietnamese Dongs (currency, 15,908 VNDs is equivalent to 1$)

Π(E, S) Profit as a function of Effort and Stocking

- vi -

ABSTRACT

Reservoir fisheries management must be based on an understanding that they are

complex and dynamic ecosystem. This study describes fisheries activities status in the

Tri An reservoir. The main objective is to determine an efficient exploitation level of

fisheries resources affected by the stocking program. The Verhulst-Schaefer model

(logistic growth) was applied and the classic model modified to also include the case

of stocking. The modified surplus production model (MSPM) that considers fish

stocking as a major factor influencing population growth has been employed to

estimate static reference points. Adding economic components to the MSPM, a

bioeconomic model was established and applied in analyzing the interaction between

human harvesting pressures, stocking and biological resources regeneration. Data on

catch/effort and stocking from 1993 to 2005 were used to analyze the fishery.

Empirical results reveal that the stocking program was a major factor influencing both

population growth and the harvest regime in the reservoir. Fish stocking was

positively correlated to change in population growth, and led to a considerable

enhancement in fish production. The fisheries resources cannot sustain current

exploitation levels which have led to both biological and economic overfishing as a

result of ineffective management. The current centralized top-down management has

proven ineffective and inappropriate. Therefore, rational management is required to

rescue the fisheries resources from depletion, to maintain the fisheries productivity

capacity and to prevent further resource degradation. However, reservoir fisheries are

currently dependent on harvesting and stocking regimes, so a change of management

plan should be achieved by simultaneously changing the level of effort and stocking

rate.

KEY WORDS: Reservoir, Stocking, MSPM, Harvest, Overfishing and Management

- 2 -

Chapter I

I. INTRODUCTION

1.1. Introduction

Reservoirs are an important water resource in Asia, the reservoir resources are diverse

and therefore the strategies to be adopted for optimizing yields are also different

(Bhukaswan 1980; Cowx 1996; De Silva 2001). Most reservoirs in Vietnam were

impounded after 1954 for different purposes such as irrigation, hydroelectricity, flood

control, and water supply for domestic consumption and industry (Hao 1997; Van &

Luu 2001). With few exceptions, these reservoirs have been used for fish production

by stock enhancement and cage culture (Luong et al. 2004). The fish production from

reservoirs depends on nutrients, biomass, and the quantity of stocked fingerlings.

There is a common belief that fish yields of reservoirs tend to be high in the initial

few years after impoundment, and then begin to stabilize at a lower level (Van & Luu

2001). Recently the fisheries resources of reservoirs in Vietnam have shown a

downward tendency, as the size and population structure of the fish species (including

stocked species) in the reservoirs have decreased (Hao 1994; Lam 1994). As

indicated by Bernacsek (1997) fisheries catch per unit effort is quite low in large

reservoirs, mainly due to the low productivity of pelagic water. Recently, reservoir

fisheries resources have tended to be overexploited (Coates 2002; Cowx 2002). First

of all this is caused by ineffective or inappropriate management measures. Open

access may be an important cause of overfishing, and lack of knowledge on fisheries

resources may lead to or result in overfishing (Coates 2002).

Tri An reservoir was built in 1987, it has an average water surface area of 250 km2,

changing from 75 to 324 km2. The main functions are hydroelectricity and

agricultural irrigation for South-east Vietnam (Hao 1997). In addition, fisheries are

beginning to be recognized as an important secondary function of the reservoir water

resource (De Silva 2001; DNFC 1997). Fishing is traditionally an important

occupation for local people living in reservoir areas and has a main role as a protein

source in the diets of many households (DNFC 1995; Hao 1997; Sonny & Oscar

2001; Tung et al. 2004). Stocking has been a major component of reservoir fisheries

management since 1995, for biological control, enhancement of fish yields and

- 3 -

employment. The stocking program leads to an increasing number of species in the

reservoir through introduction of exotic stocked fish as Tilapia, Indian carp, Grass

carp, Silver carp and Big head carp (DNFC 2005; Tung et al. 2004). Although aquatic

resources in the reservoir are highly diversified in species composition as well as

abundant primary productivity, these resources have been reduced in terms of

quantity and size of fish caught in recent years (Hao 1997; Tung et al. 2004). This

suggests that the fishery is being overexploited, and that better fishery management

needs to be imposed to maintain productivity of the fisheries resources on a

sustainable basis (Luu 1998; Sonny & Oscar 2001).

Previous studies of fisheries in Tri An reservoir focused mainly on specific technical

aspects and their specific solutions, whereas less attention was paid to other aspects

influencing the development of fisheries such as research on data archives to estimate

aquatic resources, management methods and fisheries assessment. This study,

therefore, tries to show whether or not overfishing indeed exists and the impact of

fish stocking on population dynamics. The findings of this study will contribute to fill

some of the existing gap of empirical studies focusing on the bioeconomic analysis of

sustainable use of fish resources. The thesis begins with the introduction chapter

presenting the rationale of the study, and defines the main concerns and objectives.

Background information on fisheries in the reservoir is presented in chapter 2.

Chapter 3 summarizes the basic theory and introduces a surplus production model

including the effects of a stocking program. Chapter 4 outlines the data used and steps

of data processing, followed by the results of the study in chapter 5. Finally, chapter 6

presents the discussion and conclusion based on the findings in the previous chapter.

1.2. Main concerns of the study

To carry out the study, main concerns of the fishery in the reservoir are defined as

follows:

i. The Dong Nai Fisheries Company (DNFC), a local government fishery

enterprise, is responsible for managing fisheries activities in the reservoir.

Generally, management is weak due to lack of specialized technicians and

knowledge in management. The financial source of DNFC is mainly from

the local government; with additional revenue from activities such as fish

- 4 -

hatchery, forestry exploitation and taxation on fishing. Annually, a fixed

share of fishing taxes is used for introducing fish fingerlings into the

reservoir; however, the stocking density and species rations were suspected

to be inappropriate. The stocking rate is only based on the budget of the

DNFC, instead of depending on basic stocking principles. Although

stocking was introduced as a fisheries management component 12 years ago,

studies on stocking impact on population dynamics have not been carried

out so far.

ii. As a result of ineffective or inappropriate methods of management, the

average size of fish caught and catch per unit of effort (CPUE) have showed

declining trends in recent years (FAU 2000; Hao 1997). Recent surveys

report that harvests are decreasing in terms of fish size and quantity of

commercial species. This indicates that the fish resources in the reservoir are

reduced because of overfishing. Possible reasons of resource reduction are

high fishing pressure, gears used with small mesh-size, ineffective

management and poor stocking strategies.

In order to get insight into the above problems, this study aims to assess the status of

existing capture fisheries activities and management in the reservoir. The main

objective is to identify an efficient level of fisheries resources exploitation that are

affected by the stocking program. Reference points as Maximum Sustainable Yield

(MSY), Maximum Economic Yield (MEY) and Open Access Yield (OAY),

represented by their corresponding effort and stocking levels and estimated by

applying a theoretical bioeconomic model. Based on the findings, several possible

management measures for sustainable development are discussed and recommended.

The study is based on empirical investigations that have provided insight into such

questions as: How does the fish stocking influence population dynamics and fish

harvest level? Is there overfishing in the reservoir? Is the current exploitation of

fisheries resources sustainable?

- 5 -

Chapter II

II. BACKGROUND OF FISHERY IN TRI AN RESERVOIR

2.1. Characteristics of fisheries resources

2.1.1. The location, area and climate

Tri An reservoir lies between latitudes 10000’ to 12020’ North and longitudes 107000’

to 108030’ East. The reservoir was built and completely impounded in 1987, and is

mainly used for hydro-electricity (DNFC 1997). It is also utilized for fisheries, and

has been supplying water for agricultural irrigation and domestic consumption of the

lower Dong Nai river basin. Tri An is the biggest reservoir of Vietnam, with a

catchment area of approximately 14,800 km2, an average annual outflow of 15,100

million m3 and total volume of 2,765 million m3 (see Fig. 2.1). The reservoir has a

water surface area of around 324 km2, with an average depth of 8.5 m, about 44 km in

length and has a maximum width of 10 km (DNFC 2005; Tung et al. 2004).

Figure 2.1. Map of Tri An reservoir showing the location and study area

[Source: maps cited from Tran (2002)]

The reservoir belongs to a tropical climate region, with a water temperature range of

210C-310C, and a characteristic of two distinct seasons: the rainy season from June to

- 6 -

November, with high rainfall of 2,400 mm; and the dry season from December to

June the following year (DNFC 1995; Tung et al. 2004). Additionally, the reservoir

contains about 50 coves of various sizes, and connects to many tributaries of the

Dong Nai and La Nga rivers (Hao 1997; Luong et al. 2004; Tung et al. 2004).

Therefore, the Tri An reservoir has advantageous conditions of climate and

topography for fisheries development. Although the initial principal purpose of the

reservoir was not primarily fisheries development, the fisheries value has been

recognized as an important secondary use of the water resource (De Silva 2001;

DNFC 1997).

2.1.2. Species composition

The fish species composition in a reservoir is a result of the different reactions of the

species to varying environmental conditions after impoundment (Bhukaswan 1980).

Some species not able to adapt to the new conditions may become extinct while other

species adapt to change to varying degrees and may continue to exist at a changed

abundance (Li & Xu 1995; Tung et al. 2004).

The fish fauna of Tri An reservoir first of all reflects the fauna of the impounded

Dong Nai and La Nga rivers (Li & Xu 1995; MoF 1996; Tung et al. 2004;

Welcomme 2001). The current reservoir ecosystem is diverse and includes many

species; the fish fauna is known to constitute 109 species, belonging to 28 families

and 9 orders. The Cyprinids, constituting 56 species, are still dominant among species

inhabiting the reservoir. The families of Cobitidae, Cichlidae, Siluridae and Bagridae

with 5 species of each respectively accounted for 18.35% of total, followed by

Belontiidae with 4 species; and other families were recorded with 29 species. There

are 32 commercial species and 77 low value economic species. The highly

commercial species are Common carp (Cyprinus carpio), Silver carp

(Hypophthalmichthys molitris), Big head carp (Aristichthys nobilis), Grass carp

(Ctenopharyngodon idellus) and Indian carp (Labeo rohita), and they constitute

annually the main fish production for the reservoir (DNFC 1995; Hao 1997; Tung et

al. 2004). These sources also confirm that the size and population structure of the fish

species in the reservoir have decreased recently.

- 7 -

Bighead carp (Aristichthys nobilis)

Silver carp(Hypophthalmichthys molitris)

Common carp(Cyprinus carpio)

Mud-carp (Cirrhinus molitorella)

2.1.3. Fish stocking

Stocking and introduction of fish are frequently used throughout the world (Cowx

1998; 2002; Welcomme 1998; 2001). Stocking is a management measure to enhance

and optimize yield of lacustrine bodies (Bhukaswan 1980; Li & Xu 1995). In Tri An

reservoir, stocking has been a major management component since 1995. The

stocking program was a main part of the project “Assessment of socio-economics

and investment of fisheries potential exploitation of Tri An reservoir, 1995-1999”

(DNFC 1995). The purposes of stocking are to reduce water pollution through

increasing the biological filter capacity of suitable stocked species; to utilize

ecological niches to which none of the existing species are adapted; to control aquatic

weeds; to enhance the fish yields and provide more food fish; and to curb

unemployment through fishery development (Bhukaswan 1980; Li & Xu 1995; Luu

1998). Annually, stocked fish which are re-caught contribute an average of 30% of

fish production in the reservoir (An 2001; FAU 2001).

Figure 2.2. Photographs of main stocked fish in Tri An reservoir

[Source: photographs cited from MRC (2003)]

Series of management measures for stocking in the reservoir established, including

choice of suitable species, species combinations, stocking size and measures for

preventing escape of stocked fish (An 2001; DNFC 1995; Lorenzen et al. 2001).

- 8 -

Eight fish species, including 3 indigenous and 5 exotic species, were introduced into

the reservoir (see Fig. 2.2). Recently, a change in species composition of fish in the

reservoir is a result of stocking impact, 109 species were recorded when compared

with 102 species during the pre-impoundment period (Tung et al. 2004). Some of

exotic stocked fish such as Silver carp, Big head carp, Tilapia, Indian carp and Grass

carp, lead to increase in the number of species (DNFC 2005; Tung et al. 2004).

2.1.4. Fishing community

Fisheries communities are located around or close to the reservoir at Vinh cuu, Thong

nhat and Dinh quan districts of Dong Nai province (DNFC 1997). About 1,000

households are permanently involved in fishing activities (AMCF 2002; DNFC 2005;

FAU 2000; Sonny & Oscar 2001), of which:

1. The full-time fishermen who are more specialized and operated their

fishing as a main occupation and accounted for 60% of the total.

2. The part-time fishermen who usually operated their fishing as a

consequence of lack of work in their main occupation, or who capture fish

as feed supplying for their cage culture system was about 30% of the total.

3. The subsistence fishermen who mainly harvested fish for their own

consumption occupied 10% of the total.

Fishing activities in the reservoir have supplied over 2,000 permanent employments,

and ensured livelihoods for about 1,000 households with more than 5,500 inhabitants

(DNFC 1995). The average household consisted of five to six persons with the father

being the main income earner, and consistent with the characteristics of the extended

family system, married couples tend to remain with their parents and involve

themselves in fishing activities (Ahyaudin & Lee 1994; DNFC 1995). In general,

most fishermen have a low education level, with about 17% being illiterate, 59% with

primary school level of education (FAU 2000), and their fishing practices mainly

depend on experience passed down from their forebears (AMCF 2002; FAU 2000;

Luu 1998).

- 9 -

Java barb9%

Nile tilapia12%

Grass carp9%

Mud carp13%

Indian carp9%

Common carp12%

Bighead carp12%

Silver carp 24%

2.2. Status of fish stocking program

2.2.1. Trends of fish stocking program Several species, including exotic and indigenous species, have been introduced into

the reservoir since 1995. Annually, about 1.2 million fingerlings are introduced into

the reservoir (DNFC 2005), of which planktivorous species such Silver carp and Big

head carp are dominant and accounted for 36% of total, followed by Mud carp (13%),

Common carp (12%), Tilapia (12%) and others (27%) (see Fig. 2.3).

Figure 2.3. The distribution of stocked species from 1995 to 2003

[Source: data calculated and cited from DNFC (2005)]

At the first three-year period, the DNFC received financial support from the local

government to begin the stocking program; thus, the quantity of fish stocking

increased quickly from 1.3 million fingerlings in 1995 to 5.0 million in 1997. After

that, the local government stopped financial support for the stocking program, so the

quantity of stocked fish reduced dramatically and fluctuated slightly, with an average

of 1.2 million fingerlings in the following years (see Fig. 2.4). At that time, only a

fixed share of fishing taxes was used for introducing fish fingerlings into the

reservoir, with an average of 10% of fishing taxes, however, this expenditure was not

enough to ensure fingerling quantity and to attain objectives of the stocking program

(DNFC 2005; Tung et al. 2004).

Although the stocking program was limited in terms of stocking quantity, it still led to

considerable increase in fish yield from 45 kg per ha in 1995 without fish stocking,

- 10 -

0

50

100

150

200

250

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

Yie

ld a

nd S

tock

ing

dens

ity

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Stoc

ked

fish

quan

tity

Yield (kg/ha)

Stocking density(fingerlings/ha)Stocked fish quantity(million fingerlings)

compared to over 60 kg per ha in the following years affected by the stocking

program (see Fig. 2.4). In general, the fingerlings were introduced with lower density,

around 48 fingerlings/ha/year compared with stocking principles, so re-capture rate of

stocked fish and productivity were too low to be economical (An 2001).

Recently, the DNFC has been changed it operating regime; hence, the stocking

program was stopped. The DNFC will continue introducing fish fingerlings into the

reservoir when they re-organize completely, but interruption to the stocking program

led to an immediate reduction in fish yield (An 2001; Van & Luu 2001).

Figure 2.4. Trends of fish stocking program from 1993 to 2005


2.2.2. Optimal stocking density for Tri An reservoir

Technically, estimation of maximum stocking density needs to be considered in the

process of stocking planning and management. According to Li & Xu (1995), the

theoretical equation used for stocking density estimated is:

Where d is stocking density (fingerlings/ha), F is potential

yield (kg/ha), W is average size of harvested fish (kg), and

R is re-capture rate of stocked fish (% of stocked quantity). WRFd = (2.1)

- 11 -

In Tri An reservoir, the maximum stocking density is around 600 fingerlings/ha/year,

the corresponding level of stocked fish into the reservoir is 15 million fingerlings/year

or 100 tons fingerlings/year (see Table 2.1).

Table 2.1. Estimating the maximum stocking density for Tri An reservoir

Items: Value UnitPotential yield of stocked fish (F)1 90 (kg/ha)Re-capture rate (R)2 10 (% of stocked quantity)Mean weight of fish at capture (W)3 1.5 (kg)Maximum stocking density (d)4 600 (fingerlings/ha)Average surface area (A) 25,000 (ha)Maximum stocking in: - Quantity (Q)5 15,000,000 (fingerlings) - Weight (TW)6 100,000 (kg)

1 , 2 & 3. Data are cited from Hao (1997), Van & Luu (2001) and Welcomme (1998). 4. Stocking density is maximum when re-capture of stocked fish reaches the potential yield of stocked fish, it is calculated by Eq. (2.1). 5. Quantity of stocked fish (Q) equals average surface area (A) multiplied by maximum density (d). 6. Weight of stocked fish equals Q divided by an average of 150 fingerlings per kg (DNFC 2005).

2.3. Status of capture fisheries activities

2.3.1. Fishing season

Fishing is a highly seasonal activity in most parts of the world (Welcomme 2001).

The seasonality of fishing in Tri An reservoir is determined mainly by the outflow

regime. Fishing seasons can be distinguished as dry season with low water level and

rainy season with high water level (DNFC 2005), as follows:

i. Dry season (from January to June) is main fishing season, and the highest

fishing intensity starts from April to the end of May. Trawl net, gillnet, long

line, big cast net and beach seine are widely used, and their catches are

larger than for other gear types. The fishing grounds are mainly

concentrated at the middle and the lower basin of the reservoir.

ii. Rainy season (from July to December) is the supplemental fishing season.

Fishing gears widely used with high catches are the sprat scoop net, shrimp

basket trap, scoop net and gillnet. In this season, the water surface area may

vary from 25,000 ha to 32,400 ha, hence, most fishing grounds are used.

- 12 -

2.3.2. Fishing gears The reservoir capture fishery reflects the general state of inland fisheries in Southeast

Asia by being multi-species and multi-gear fisheries, primarily artisanal and small-

scale (Ahyaudin & Lee 1994; Coates 2002). The types of fishing gear used depend

mainly on the habitats exploited, fishing season, the target species and the purpose of

exploitation (AMCF 2002). The DNFC divided these gears into 17 categories (see

Appendix 2). The fishing gears widely used are gillnet 1 (average of 28% of total

households use), followed by shrimp basket trap (29%), gillnet 2 (8%), sprat scoop net

(7%) and long line (6%) (see Table 2.2 and Appendix 4). They also contribute mainly

and largely to the total fish production of the reservoir (DNFC 2005). The use of

illegal gears and prohibited destructive fishing activities like the use of explosives,

toxicants (poisoning), and certain other destructive fishing methods such as filtering

barrier with small mesh size, have been banned since 1995. However, many fishermen

still operate these types of prohibited activities (Bhukaswan 1980; DNFC 2005).

Table 2.2. Major fishing gears used in Tri An reservoir

Gear name Description

% of

householdsused

% of total catch

Average catch

(kg/year) Gillnet 1 - a net 1,000 metres in length/gear

- the mesh size ranges 40–60mm 16

÷4013

÷33 1,448

÷2,521Gillnet 2 - a net 1,500 metres in length/gear

- the mesh size ranges 70–140mm 4

÷104 ÷8

710÷3,251

Long line - a long-line 1,000-1,500 hooks 2÷9

1 ÷3

527÷2,538

Scoop net - a scope net with 1 oil-lamps/gear 4÷5

5 ÷24

3,101÷16,542

Shrimp basket trap

- It is made by bamboo. - a 100 basket-traps/boat

24÷34

5 ÷9

334÷914

Sprat scoop net

- a scope net with 18 lamps/gear - the mesh size ranges 2–5mm

5÷12

28 ÷32

5,405÷17,151

Trawl net - a net 500 metres in length/gear - the mesh size ranges 70–150mm

2÷5

2 ÷5

1,182÷7,088


- 13 -

0

500

1,000

1,500

2,000

2,500

3,000

3,500

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

Cat

ch

0

200

400

600

800

1,000

1,200

1,400

1,600

Effo

rt

Catch (tons)

Effort (numberof households)

2.3.3. Fish production and market

Capture fisheries in the reservoir have increased both in fish production and in effort

over a period of ten years (1993-2002). The growth rate of fish production rose

quickly as a result of introducing fish fingerlings into the reservoir, and increasing

effort (DNFC 1997; 2005). Recently, both production and effort were reduced, from

2003 up to now (see Fig. 2.5). The reasons for this are the interruption of the stocking

program when the responsible fishery company is changing the operating regime, and

weakness in fisheries management. Declining profit of fishing activities is the main

reason for many fishermen leaving the fishery (AMCF 2002; DNFC 2005; FAU

2000). Average of approximately 2,500 tons fish is landed annually, representing

about 80% of the total fish production of Dong Nai province. Fishing activity is also

an important occupation of local people living in the area close to the reservoir;

giving permanent jobs for over 1,000 households and more than 2,000 employees

(DNFC 2005; Tung et al. 2004).

Figure 2.5. Trend of catch and effort from 1993 to 2005

[Source: data cited from DNFC (2005)]

Fish landed are transported from the fishing grounds to six commercial landing sites

close to the reservoir, each controlled and operated by about 10 permanent

wholesalers and a landing centre of local government through various middlemen.

- 14 -

Most fishermen sell their harvest to the middlemen. The relationship between

middlemen and fishermen has been set up for a long time. Middlemen often buy

catches from fishermen at low prices, while the middlemen on their side ensure the

fishermen that all their catches will be bought and also provide small loans to

fishermen if needed (Ahyaudin & Lee 1994; DNFC 1995; 2005; Sonny & Oscar

2001). Price on harvest is mostly decided by middlemen when fishermen have a debt

to them. Landed fish prices tend to be lower in the dry season compared with the

rainy season, because of differences in available catches and productivity between dry

and rainy seasons (DNFC 2005; Hannesson 1993). Many fishermen sell their product

directly to retailers and consumers at market prices in order to achieve higher profits.

Harvested fish with low economic value as trash fish are often sold directly to owners

of cage culture farms to be processed and supplied as a fish feed source for cage

culture (DNFC 2005).

2.4. Fisheries management practices

After impoundment, the DNFC is responsible for running the hatchery, producing

fingerlings for stocking and managing fisheries activities. The usual management

problems related to ownership are therefore avoided. However, production of a plan

is initiated by the DNFC depending on its investment capacity, availability of

financial resources and marketing. The problem of controlling illegal fishing is

substantial, being one of the most important social issues in reservoir fisheries. In

addition, property-sharing in water resources among the DNFC and other

stakeholders such as Tourism and Industrial Companies has produced conflict (Luu

1998; Van & Luu 2001).

A series of fisheries management measures employed in the reservoir was set up by

the DNFC in 1995, including introduction of fisheries regulations, fish stocking

strategies and monitoring strategies (DNFC 1995; 1997; FAU 2000).

2.4.1. Fishing regulations

The principal purpose of fishing regulations and control is to ensure a high, but

sustainable yield (Bhukaswan 1980). Decision No 171/QD-UBNDTDN on 1 August

1995 (DNFC 1995), which describes the legal framework regarding the exploitation

- 15 -

of fisheries resources in the reservoir, comprises mainly protection and conservation

measures to control fishing such as:

i. Technical measures: the DNFC set up management measures such as closed

area and closed season which are to establish localities and times of the year

when fish must not be taken to protect the brood-stocks and fry fish/or

fingerlings in spawning seasons/areas (Charles 2001; Welcomme 2001).

However, where the fishing depends on multi-species resources, it is

difficult to design appropriate closed season and closed area regulations to

ensure adequate protection for all species (Bhukaswan 1980; De Silva

1988), the DNFC suggested closed seasons and closed areas for 10 high

value economic species. Limitations on minimum mesh size of fishing gears

were also established. Pollution regulation and control of prohibited wastes

release into the reservoir are enforced for all stakeholders using the water

resources. Partitioning off the reservoir with a barrier net fixed across those

cove mouths which are spawning grounds or passage for migration in

spawning season is prohibited for all fishermen (DNFC 1995).

ii. Input control: although the capture fishery in the reservoir is open access

due to local community pressure, entry to the fishery is restricted only to the

local people. All local fishermen can access fishing, however, they have to

sign a contract to license/or pay operation fees with the DNFC before

entering the fishery. Based on type of gears used and operating time,

fishermen have to pay operation fees to the DNFC (DNFC 1995).

iii. Output control: the minimum size regulation on fish caught is considered to

be an important control measure, particularly for species with low

reproductive capacity. As a result of the difficulties in setting up proper

minimum size regulation in a multi-species fishery (Bhukaswan 1980; De

Silva 1988), the DNFC only identified and implemented size limitations to

protect about 20 commercial species. The size limitations of fish caught are

enforced for both fishermen and wholesalers. In particular cases, if

fishermen want to harvest fry fish/or fingerlings used for cultivation in

coves and cage culture systems, the fishermen have to apply for a license

and will be controlled by managers from the DNFC (DNFC 1995).

- 16 -

Although fishing regulations were implemented in 1995, the effects have been limited

and it has been difficult to achieve targets due to weakness of management capacity

and low awareness level of fishermen about conservation issues (Luu 1998).

2.4.2. Fish stocking strategies

Fish stocking has proven to be one of the most successful tangible tools in reservoir

fisheries management (An 2001; Li & Xu 1995). The DNFC set up strategies for the

stocking program as follows:

i. Choice of suitable species, species combinations, stocking size and rate, and

measures for preventing escape of stocked fish were studied. Stocked fish

feed on phytoplankton, zooplankton, organic detritus and periphyton to be

of priority. Stocked fish must be well and minimum size of fingerlings is 8-

12 cm in length (An 2001; DNFC 1995; Luu 1998).

ii. Annually, about 2-3 million fingerlings have been introduced into the

reservoir, with an average density of 100 fingerlings/ha. The stocking time

is from August to December, and the harvesting time of stocked fish is from

February the following year. The DNFC also chooses suitable areas for

stocking, and establishes the closed areas and seasons for all fishing gears

(DNFC 1995; 2005; Luu 1998).

iii. To ensure the stable and annual budget source for the stocking program, all

fishermen who enter the fishery have to pay operation fees. The fee size

varies by type of gears and intensity level from 10 to 20% of the value of the

total catch. This taxes source will be used to control fishing activities and

stocking at the following year (DNFC 1995; 2005).

Overall, because of ineffective management and financial constraints, the above

stocking regulations could not be implemented correctly and it is difficult to obtain

clear objectives of the current stocking program. Annually about 10% of fishing taxes

was used for stocking, hence, the stocking density was low leading to low recapture

rate and ineffectiveness of the stocking program (An 2001; Van & Luu 2001).

- 17 -

Chapter III

III. MODEL

The abundance of fish stock in a particular area is a function of interactions between

environmental factors and the fish stock properties. The stock tends to stabilize at a

particular set of environmental conditions (Gulland 1977). When the surplus

production is not harvested, at the level of maximum fish stock size the addition of

recruitment and growth to the stock is just sufficient to compensate for natural

mortality and hence, surplus production will equal zero (Clark 1990; Haddon 2001;

Hannesson 1993). This implies that fishing plans can be expressed in terms of surplus

production. The surplus production models are very flexible and have different

variations; Verhulst-Schaefer, Gompertz-Fox and Pella-Tomlinson models are some of

the best known and popular (Frank et al. 1979; Seijo et al. 1998). In this study a

modified surplus production model that was developed from Verhulst-Schaefer model

has been built and used for assessment of fish stock affected by the stocking program.

3.1. The logistic growth model

Population growth has been typified in several ways, but most commonly, the logistic

model of population growth has been found to fit a large number/or stock biomass of

populations both in nature and in captivity. Generally, the Verhulst (1838)/Pearl

(1925) surplus production model (SPM) was defined as change in population biomass

per unit of time (Clark 1990; Flaaten 2004), and is described by the logistic equation:

Where X is stock biomass, and r and K are positive constants referred to as the intrinsic

growth rate and environmental carrying capacity, respectively. The reason for these

terms is simple: r represents the maximum relative growth rate of population, which is

the approximate rate of growth, when X<<K. Similarly, K represents the stable

equilibrium biomass level: if X is a positive solution of Eq. (3.1), then X → K as t → ∞

(Clark 1985). F(X) is natural population growth describing change in population biomass

per unit of time. Natural population growth is usually positive, but may even be negative

if the stock level for any reason is higher than K. F(X) has its maximum for a specific

dtdX

KXrXF X =

−= 1)( (3.1)

- 18 -

stock level that may be found by maximizing F with respect to X, at which the

productivity of resource is maximum (Clark 1990; Flaaten 2004).

In order to apply the logistic growth model to fish population dynamics that considers

the effect of fishing (Clark 1985; Frank et al. 1979; Gordon & Clark 1980), Schaefer

model (1957) is the most commonly used among SPMs (known as Biomass Dynamics

Models), it bases precisely on the logistic population growth model of Verhulst

(1838)/Pearl (1925). To include the effect of fishing in Eq. (3.1), Schaefer introduces

the rate at which fish are caught, that is, the catch rate H of the fishery, is given by:

Where H denotes the catch rate in terms of fishing effort E, and q is a constant called

catchability coefficient and X is the fish stock biomass.

When catch/or harvest (Eq. 3.2) is included in the Eq. (3.1) in order to model the

effect of fishing on fish population dynamics, Schaefer (1957) altered Eq. (3.1) to

While the Verhulst-Schaefer model (Eq. 3.3) is desirable to utilize the fish resources, it

is intuitively clear that if many fish are harvested, then the fish population may be

reduced below a useful level, and possibly even driven to extinction (Boyce & DiPrima

1992). Thus, the sustainable harvest of fish resources generally requires that the catch

rate should not exceed the growth rate of fish stock (Charles 2001). Setting dX/dt=0 is

the condition of a sustainable equilibrium in the Verhulst-Schaefer model, this model

helps to find MSY of the fish resources, corresponding to the equilibrium value

X=K/2. This indicates that the fishery is unsustainable if the catch rate exceeds MSY.

The fishery is considered to be overexploited if the stock is reduced to a level below

K/2 or sustainable yield falls below MSY (Boyce & DiPrima 1992).

3.2. Modified surplus production model

In practice, fish fingerlings are frequently introduced into reservoirs, thus fish stocking

directly influences fish population dynamics (Cowx 2002). The previous studies for

reservoir fisheries show that sexual maturation of stocked fish under natural conditions is

impossible or very low, and stocked fish are unable to reproduce locally or to adapt and

HKXrX

dtdX

−

−= 1 (3.3)

qEXH = (3.2)

- 19 -

dtdX

KXXaSrF SX =

−+= 1)( 0),( (3.4)

compete successfully with local species in terms of growth and competition for food

(Ali 1998; Hao 1997; Welcomme 1998). Therefore, stocking is the principal method

used in reservoir fisheries to enhance fish production, to maintain fisheries

productivity or to improve recruitment and bias fish assemblage structure which is a

result of overfishing (Cowx 2002; Welcomme 2001). Regarding the stock assessment

of the reservoir, the Verhulst-Schaefer model can be applied in order to provide initial

reference points for management in the reservoir, however, the SPM needs to be

modified due to the stocking program’s impact on fish population growth. The

following modification is proposed:

Where (r0 + aS ) is intrinsic growth rate affected by stocked fish S, and F(X, S) is population

growth, it is the change in population biomass per unit of time affected and depended on

stocked fish, in the case of no fishing. When S=0, Eq. (3.4) is reduced to Eq. (3.1).

Cowx (1998) and Welcomme (1998) report that when stocked fisheries have been

managed well, usually in terms of stoking strategies and fishing regulations, the

stocking could help to maintain the fisheries productivity at the highest possible level.

Furthermore, the stocking may help to compensate for recruitment overfishing,

because the stocking may provide sufficient quantity of catch demands to increasingly

higher fishing pressure levels (Cowx 2002). It is reasonable to assume that the rate at

which fish are harvested depends on the fish population: the more fish stock including

wild fish and stocked fish there are, the easier it is to catch them (Boyce & DiPrima

1992). Thus, the stocking can help to reduce fishing pressure for natural fish stock,

and the fish stock may increase as positive results of the stocking.

In this point of view, we assume that the stocking is positively related to population

growth through intrinsic growth rate of population (r); it means that r will increase

when fingerlings are introduced into the reservoir, because stocked fish are unable to

reproduce and they help to reduce fishing pressure for natural fish stock. This indicates

that the fish population growth affected by stocked fish will increase and approach the

carrying capacity of the environment more quickly compared to change with no stocking

(King 1995; Lever 1998) (see Fig. 3.1). As the result of the change in intrinsic growth

rate affected by fish stocking, the population growth tends to be positively trended with

- 20 -

05000

1000015000

20000 0

20

40

60

80

100

02000400060008000

05000

1000015000

K

S-stocking

X-stock biomass

F-population growth

02

46

8Time

0

20

40

60

80

10

Stockin

0

000

0

02

46

8Time

K

0 0

10 0

100

S-stocking

Time

Xt-population size

stocking rate, it rises when stocked fish increase and vice versa (see Fig. 3.2).

Figure 3.1. Population growth according to the logistic curves

The graph shows the trajectories of population growth for the different values of r.

Integrating Eq. (3.4) leads to the continuous solution to the logistic equation, giving the

expected population size Xt at time t after some starting time and population size X0 ,

( ) ( )( )[ ]aSrt eXXKKX +−+= 0

00 /1/ . The top dashed line represents the asymptotic

carrying capacity K. Maximum growth rate is at K/2, where the inflection exists in the

curve. The population size affected by stocked fish will increase and approach the

environmental carrying capacity more quickly compared to change with no stocking.

Figure 3.2. Plot of population growth versus stock biomass and stocking rate

The graph shows the relationship of population growth with stocking and stock levels.

Population growth is a function of stock and stocking rates, F(X,S) = (r0 + aS)X[1-X/K]

- 21 -

01

23

45 0

20

40

60

80

100

0

2000

4000

6000

01

23

4

0 0

0

100

S-stocking

E-effort

H-harvest

12

In this case, in order to model the effect of fishing on fish population dynamics, the

catch rate H (Eq. 3.2) is included in the Eq. (3.4), we obtain the MSPM as:

Sustainable equilibrium of the population dynamics model occurs when setting

dX/dt=0; the MSPM helps to find the MSY of the fish resources exploitation in the

reservoir, it indicates that the catch rate should not exceed the MSY level to ensure

sustainable harvesting, and the fish resources are overexploited when sustainable

yield is reduced to a level below MSY.

Replacing Eq. (3.2) into Eq. (3.5) and rearranging Eq. (3.5), the stock biomass can be

expressed in equilibrium condition as:

Substituting Eq. (3.6) into Eq. (3.2) and rearranging Eq. (3.2), the harvest rate is

estimated in equilibrium condition as:

Figure 3.3. Plot of harvest curve versus effort level and stocking rate

The graph is plotted from Eq. (3.7), and shows relationship of harvest rate with effort and

stocking. At a given stocking rate, Eq. (3.7) is quadratic in E, the effort is only positively increase

in harvest rate up to a certain limit, after which yields decline. Whereas, at a given effort level

Eq. (3.7) is a line up in S, the stocking is positively correlated to harvest rate.

+

−=aSr

qEKX SE0

),( 1 (3.6)

+

−=aSr

qEqEKH SE0

),( 1 (3.7)

HKXXaSr

dtdX

−

−+= 1)( 0 (3.5)

- 22 -

We assume the constant q (i.e. no changes in gear or vessel efficiency have taken

place), and biologically it implies that environmental conditions are constant/or no

environmental factors affect the population. The intrinsic growth rate affected by

stocked fish responds instantaneously to changes in biomass. The CPUE, a direct

(proportional) index of stock abundance X(E,S), can be expressed as:

3.3. Biological analysis

The SPM which utilizes catch and effort data has been used widely in fisheries

management (Clark 1985; King 1995). Biological analysis help managers to know

levels of change in population biomass per unit of time, the MSY is introduced as the

simplest management objective that shows the level of biological stock not being

exploited too heavily without an ultimate loss of productivity (Clark 1990). Applying

annual effort level EMSY and stocking rate SMSY will produce the maximum harvest of

fish that can, in theory, be caught year after year indefinitely into the future.

The MSY can be obtained when partial derivative of harvest function in the long run

(see Eq. 3.7) that depends on effort or stocking variables equals zero, as follows:

The EMSY can be estimated based on Eq. (3.9). While, )(SH•

in Eq. (3.10) is always

positive or )(SH•

0 when S + ∞. The harvest is positively related to stocking rate

(see Fig. 3.3), however stocking rate is technically and biologically limited in terms

of stocking density. In context of stocking practices, technically the SMSY cannot

exceed a given maximum stocking quantity (see Table 2.1). The Eqs. (3.11) and

(3.12) indicate that EMSY is a function of SMSY and vice versa, thus they cannot be

solved separately. We carried out the numerical solution by Mathematica 5.2, the

SMSY is estimated and the EMSY is also solved with a given range of stocking quantity.

Inserting the EMSY and SMSY into Eq. (3.7), one has the local maximum yield.

+

−=aSr

qEqKCPUE SE0

),( 1 (3.8)

qaSr

E MSYMSY 2

0 += (3.9)0)(

2

0

2

)( =+

−=•

aSrKEqqKH E

0)( 2

0

22

)( =+

=•

aSrKqaEH S (3.10) a

rqES MSY

MSY02 −

=

(3.11)

(3.12)

- 23 -

3.4. Economic analysis

In order to analyze the interaction between human harvesting pressures, stocking and

biological resource regeneration, economic analysis is used in the study of fisheries

exploitation (Clark 1973; Lokina 2000; Seijo et al. 1998). Economic analysis can help

managers to answer the questions of why resources are used as they are, why fisheries

are economically inefficient, and how fisheries could be better managed (Hannesson

1993; Jennings et al. 2001).

The total fishery cost and total revenue are essential in an economic analysis. It is

assumed that the objective is to maximize the resource rent of the fishery. Cost per

unit of effort C1, cost per unit of stocking C2 and harvested fish price p are constants

being used for economic analysis. In the long run, we assume that vessels are

homogenous with respect to cost and catchability, C1 is constant and equal for all

vessels. The reason for this is the long run perspective where it is reasonable to

assume that adding homogenous vessels to the fleet can expand effort at a constant C1

(Flaaten 2004). The C2 varies slightly among types of stocked species, thus we also

assume that C2 is a constant. The p is largely dependent on the quality of landed fish

that is mixed fish, however, mixed fish caught should be treated as an aggregated fish

stock in order to estimate an average fish price, and let us assume that p is constant.

Introducing a bioeconomic model, in which:

Total cost TC is the sum of cost for effort and cost for stocking, it is a function of

effort and stocking:

Fish price multiplied by harvest rate in Eq. (3.7) gives the total revenue TR, it is a

function of effort and stocking:

The economic rent/or profit ∏ is the difference between TR and TC, it is also a

function of effort and stocking:

TC(E,S) = C1E + C2S (3.13)

( )SCECaSr

qEpqEKSE 210

),( 1 +−

+

−=∏ (3.15)

+

−=aSr

qEpqEKTR SE0

),( 1 (3.14)

- 24 -

01

23

4 0

20

40

60

801

000

01

23

E-effort

S-stocking

П-profit

10

012

0

0

Figure 3.4. Plot of profit curve versus effort level and stocking rate

The graph is plotted from Eq. (3.15), and shows the relationship of fishing profit with effort and

stocking. At a given stocking rate, Eq. (3.15) is quadratic in E, the effort is only positively

increase in the profit up to a certain limit, after which the profit declines. Whereas, at a given

effort level Eq. (3.15) is line in S, the stocking is mostly positive trend to the profit. However, if

high effort level combined with low stocking rate and vice versa will produce negative profit,

thus the fishing profit is managed by simultaneous control on both stocking rate and effort level.

3.4.1. MEY, the corresponding effort and stocking rate

Regarding economic efficiency in the fishery, an aspect that needs to be considered in

the harvesting of fish resources is the maximization of the economic rent (Lokina

2000). That refers to attaining the economic equilibrium, which is referred to as the

MEY. It is important not only because it protects fish stock and guarantees

sustainability, but also because it assures that resources will be allocated to the fishery

correctly and in a way that maximizes the returns from fishing (Kompas 2005).

Setting annual effort level EMEY and stocking rate SMEY generate theoretically a

maximum level of sustainable economic rents from the fishery, obtainable each year

indefinitely into the future. The MEY is attained when the partial derivative of profit

function (Eq. 3.15) that bases on effort or stocking variables equals zero as follows:

201

2))((

KpqaSrKpqC

E MEYMEY

++−= (3.16)021

01)( =

+

−+−=∏•

aSrqEpqKCE

0)( 2

0

22

2)( =+

+−=∏•

aSrKpqaECS (3.17)

2

20

Ca

CrpKaqES MEY

MEY

−=

(3.18)

(3.19)

- 25 -

The EMEY can be estimated by Eq. (3.16), and solving Eq. (3.17) gives two values of

SMEY but the negative value is excluded. The EMEY is a function of SMEY and vice

versa. In general, to obtain MEY, the EMEY and SMEY cannot be solved separately, and

then one value of EMEY and SMEY respectively has been determined by the numerical

solution from Mathematica 5.2. As mentioned above, technically the SMEY cannot

exceed a given maximum stocking quantity (see Table 2.1). Thus, the SMEY is

estimated with this range, and the EMEY is also calculated. The local maximum yield

is estimated by inserting EMEY and SMEY into Eq. (3.7).

3.4.2. OAY, the corresponding effort and stocking rate

Fisheries based on biological highly productive resources with large r and K, may

sustain a large fishing effort under open access (Flaaten 2004). In some cases,

stocking is to develop a largely culture-based fishery while maintaining high

exploitation rate, it helps to curb unemployment through fishery development

(Lorenzen et al. 2001). Thus, the stocked fisheries may take place in an open access

system. Applying annual effort level EOAY and stocking rate SOAY will still produce

normal profits for the fishermen but zero economic rents from the fishery. The OAY

is obtained when total cost (Eq. 3.13) equals total revenue (Eq. 3.14) as:

Solving Eq. (3.20) gives two values of EOAY and SOAY respectively, and the small

values are excluded because the main purpose of an open access system is maximum

employment (see Fig. 3.5). The EOAY is a function of SOAY and vice versa, thus EOAY

and SOAY cannot be solved separately. Technically, the SOAY cannot exceed a given

SCECaSr

qEpqEK 210

1 +=

+

− (3.20)

++−+−

= 21001

2KpqaKpqSSaCKpqrrC

E OAYOAYOAY

( ) ( )2

2202

221001

24

KpqSaCSrCKpqaKpqSSaCKpqrrC OAYOAYOAYOAY +−−+−

+

(3.21)

−−+−

=2

021

2aCrCKpqaEEaCS OAYOAY

OAY

( ) ( )2

00122

22

021

24

aCKpqrErECkpqEaCrCaeKpqEaC OAYOAYOAYOAY +−−+−+−

−

(3.22)

- 26 -

maximum stocking quantity (see Table 2.1). The EOAY and SOAY can be calculated by

the numerical solution from Mathematica 5.2, with a given range of stocking

quantity. The OAY is achieved by inserting EOAY and SOAY into Eq. (3.7).

Figure 3.5. Plotted curves of TR, TC vs. effort and stocking

The graph is plotted from Eqs. (3.13) and (3.14), and shows the relationship of total cost and

total revenue with effort and stocking. The TC curve intercepts the TR curve at two points, but

only the intercepted point that produces the higher effort level and stocking rate is selected

because of maximum fishing employments in open access systems.

0

2

4

6

Effort

0

25

50

75

100

Stocking

0

20000

40000

TR

0

2

4Effort

12 0

0 0

100

S-stocking

E-effort

TR TC

TC

TR

- 27 -

Chapter IV

IV. DATA AND PARAMETERS ESTIMATION

4.1. Type of data To carry out the study, time-series of data (1999-2005), which is considered more

accurate and sufficient to set up a detailed modified surplus production model, was

gathered on the following variables catch, effort, harvested fish price, stocking and

fishing cost. Data on catch, effort and cost of stocking were collected from the

DNFC, and the fishing cost data came from the survey in July 2005 and cited from

research in 2004 of Agriculture and Forestry University.

4.1.1. Catch and effort data

The DNFC has recorded catch and effort data through the Fisheries Tax Office and

surveyed fishing data (see Table 4.1). The DNFC has annually implemented

collection of statistics for catch/effort data by types of fishing gears, and fishing

effort data have been recorded and being represented by number of households,

boats and days of fishing (see Appendix 2).

Table 4.1. Actual catch and efforts data of capture fisheries, 1993-2005

Year Catch Type of fishing effort (tons) No.of fishing

households No.of fishing

boats No.of fishing

days 1993 800 300 - - 1994 833 400 - - 1995 1,126 550 - - 1996 1,475 748 - - 1997 1,825 800 - - 1998 1,840 1,234 - - 1999 2,269 1,136 992 147,602 2000 2,301 1,470 1,470 143,363 2001 2,786 1,237 1,020 125,840 2002 3,118 892 822 162,065 2003 3,080 978 978 165,354 2004 2,835 898 898 148,937 2005 2,589 872 809 132,520


- 28 -

4.1.2. Cost of fishing effort data

Operating costs reflect method, intensity of fishing effort and the amount of capital

invested in the study site. Two types of cost for fishing activities are in Table 4.2, as

follows:

1. The fixed costs are calculated in terms of the payment for fishing tax and

depreciation of both fishing boat and gear used in Tri An reservoir; and

2. The variable costs include the costs of bait, energy, cost for gear and boat

repairing, other accessories and actual labors costs.

All cost data were collected from 116 surveyed fishing households in 2004 of

Agriculture and Forestry University (Vietnam) and 22 surveyed fishing households in

July 2005. The average cost of fishing was 17.26 million VNDs/year for a fishing

household operation (see Table 4.2).

Table 4.2. Average investment fishing costs of households (2004-2005)

Type of costs:

Value(million VNDs/household)

% of total cost

Fixed costs: Depreciation of boat 0.60 3.47Depreciation of gear 2.60 15.08Operating tax 2.21 12.83

Variable costs: Operating costs 9.29 53.82Labors 2.55 14.80

Total cost 17.26 100.00

[Source: data calculated and cited from FAU (2004) and Survey in July 2005]

4.1.3. Harvested fish price data

Harvested fish price data have annually been recorded from 1999 to 2005 through

the Fisheries Tax Office and annual fisheries statistics by 17 types of fishing gear

from the DNFC. However, fish price depends on species caught and species group,

some of them are very expensive and another is low (see Appendix 2).

- 29 -

4.1.4. Fish stocking data

Fish stocking data, containing quantity and weight of stocked fish, fish

composition and fish stocking cost, has been recorded since 1993 through the

Technical Fisheries Office of the DNFC (see Table 4.3 and Appendix 1). The cost

per unit of stocked fish C2 was about 25,000 VNDs per kg of fingerlings/or 150

fingerlings, therefore, the total cost of stocking is the cost of all individual stocked

fish multiplied by cost per unit of stocked fish (DNFC 2005).

Table 4.3. Fish stocking activity in Tri An reservoir, 1993-2005

Year Number of Fish stocking species

stocking Quantity

(fingerlings) Weight

(kg) Total cost

(million VNDs) 1993 0 0 0 01994 0 0 0 01995 8 1,300,000 8,827 220.681996 8 1,900,000 12,900 322.501997 8 5,006,000 33,986 849.651998 8 1,000,000 6,789 169.731999 8 1,317,000 8,941 223.532000 6 1,200,633 7,233 180.832001 8 1,500,165 11,056 276.402002 8 1,168,705 8,178 204.452003 8 868,348 5,698 142.452004 0 0 0 02005 0 0 0 0


4.2. Data analysis

4.2.1. Effort standardization

To get one measure of fishing effort for the annual total catches, the effort values

from individual gear type had to be converted into standard units of effort. According

to Mark & Andre (2004), various methods for standardizing catch and effort data

have been developed by Gulland (1956), Beverton and Holt (1957), Robson (1966),

and Honma (1973). However, the approach developed by Beverton and Holt (1957)

was commonly applied, this method involves selecting a ‘standard vessel/gear’ and

determining the relative fishing power of all other vessels/gears (Beverton & Holt

1993).

- 30 -

In this study, fishing effort is described in terms of the number of fishing days, number

of boats and number of households used associated with types of fishing gears. The

data showed that the fishing gears used most frequently are the sprat scoop nets,

gillnet 1, gillnet 2 and shrimp basket traps. The gillnet 1 is selected as standardized

gear for 17 types of fishing gear. There are several criteria to select this gear, as

follows:

i. Gillnet 1 is a main fishing gear, and being used as commercial and full-time

fishing in the study site;

ii. The number of households and boats use of gillnet 1 is higher;

iii. The composition of fish caught by gillnet 1 is the most diversified; and

iv. Gillnet 1 contributes higher catches to fish production of the reservoir.

The fishing effort is measured in days of fishing, and the effort values from individual

gear type are converted into standard effort unit, the “gillnet 1”. The set of fishing

gears are labeled from 1 to 17, and the total catches of each fishing gear is H1,

H2,...H17, respectively, and the corresponding levels of fishing effort are E1, E2,...E17.

Therefore, the CPUE of fishing gear i is defined as:

Where CPUEi is catch per unit of effort of gear i, Hi is catch of gear i, Ei is effort of

gear i, and i is type of fishing gear used.

The Gillnet 1 denoted as gear number 1 being chosen as the standard fishing unit in this

study. The stock was assumed to follow logistic growth, then a year-by-year

procedure is used to obtain the standardized effort values, and then total standardized

fishing effort can be calculated as:

Where ES is total standardized effort, E1 is effort of gillnet 1, Ei is effort of gear i,

CPUE1 is catch per unit of effort of gillnet 1, CPUEi is catch per unit of effort of gear

i, and i is type of fishing gear used.

ii

iS E

CPUECPUE

EE ∑=

+=17

2 11 (4.2)

i

ii E

HCPUE = (4.1)

- 31 -

Table 4.4 shows the converting factors for dominant gears and the effort

standardizations respectively; whereas cast nets, spears, seine nets, lift-nets and small

traps were also treated and presented in the same group “others” because of less

important and less frequent using in the reservoir.

Table 4.4. Effort standardizations by type of fishing gears from 1999 to 2005

Gears: Items: 1999 2000 2001 2002 2003 2004 2005

Actual effort1 77.13 56.35 32.33 31.25 40.58 38.04 35.50Factor2 1.00 1.00 1.00 1.00 1.00 1.00 1.00

Gill net 1 Std. effort3 77.13 56.35 32.33 31.25 40.58 38.04 35.50


Gill net 2 Std. effort3 16.22 5.99 10.09 14.81 20.15 20.08 20.01


Long line Std. effort3 1.72 5.14 6.04 7.82 10.10 9.60 9.11


Scoop net 1

Std. effort3 12.89 16.64 46.55 51.70 70.87 67.76 64.67Actual effort1 23.20 46.14 27.25 52.85 48.25 43.63 39.00Factor2 0.49 0.31 0.32 0.37 0.53 0.49 0.44

Shrimp basket trap Std. effort3 11.35 14.48 8.76 19.70 25.45 21.22 16.97


Sprat scoop net Std. effort3 75.27 48.23 58.63 78.55 90.25 82.95 75.63


Trawl net Std. effort3 12.52 3.32 5.71 8.66 11.19 11.79 12.40

Actual effort1 10.65 10.03 17.54 22.56 21.45 19.27 17.09Factor2 0.31

÷4.690.22

÷3.620.31

÷4.020.92

÷3.580.84÷6.98

0.83 ÷7.01

0.78÷3.82

Others4

Std. effort3 28.69 20.05 25.25 33.71 45.57 38.88 32.15Total standard effort 235.8 170.2 193.4 246.2 314.2 290.3 266.4

1. Data are calculated from data presented by DNFC (2005), with unit in 1,000 fishing days. 2. Converting factors estimated base on gillnet 1 as standard gear, and Eqs. (4.1) and (4.2). 3. Data estimated base on Eqs. (4.1) and (4.2), with unit in 1,000 fishing days. 4. Present other gears with converting factor varied largely from 0.31 to 7.01.

- 32 -

100i

iii C

CRIR

−= (4.3)

iS

i

EC

C =1 (4.4)

4.2.2. Cost of fishing effort and fish price estimation We assume that the gears/vessels are homogenous with respect to cost and

catchability, because in the long run adding homogenous gears/vessels to the fleet can

expand effort when setting a constant cost per unit of effort C1 for all gears/vessels

(Flaaten 2004).

A cost-efficiency analysis of all gears can be implemented to select the most cost-

efficient. The interest rate of capital investment may be evaluated to find the fishing

gear that produced the most cost-efficient, and it can be expressed as:

Where IRi is interest rate of capital investment, Ri is total revenue of gear i , and Ci is

total cost of gear i operating that includes fixed costs and variable costs.

The cost per unit of effort for this study can be estimated as:

Where Ci is total cost of gear i that can be produced the most cost-efficiently, and EiS

is effort standardization that is converted into standard units of the “gillnet 1”.

Table 4.5 shows that “Seines net 1” is produced in the most cost-efficient manner,

therefore its cost per unit of effort can be selected for this study. The cost per unit of

effort for fishing in Tri An reservoir was 64,867 VNDs.

The average harvested fish price of “Seines net 1” was 12,429 VNDs/kg. However,

this price cannot be attained for aggregate fish caught that are multi-species, and may

not be applied for this study, some reasons are:

i. The “Seines net 1” often harvests the big fish, with a range of mesh size from

4 to 14cm. Thus, price of harvested fish was always higher than that of others.

ii. The fish fauna in the reservoir are multi-species resources, so the price of

harvested fish varies largely and depends on species caught in terms of

harvested size, fish quality and type of fishing gear used.

iii. According to fishermen and wholesalers interviewed, they indicated that the

average of aggregate fish caught price was generally around 5,500 VNDs/kg.

- 33 -

The results of cost-efficiency analysis show the average interest rate was always

positive for all gears (see Table 4.5). Mixed fish of harvested fish should be treated as

an aggregated fish stock, hence, in order to estimate an average price of harvested fish

for this study; the weighed data method was applied and expressed as:

Where p is the average of harvested fish price, i is type of fishing gears, PGAi is the

fish price of gear i, and TCC7i is total catch in 7 years of gear i (data: 1999-2005).

Based on Eq. (4.5), the average price of landed fish in Tri An reservoir was 5,485

VNDs per kg (see Appendix 2).

Table 4.5. Cost-efficiency analysis by fishing gears

Type of gears1 Total Cost2

Total Revenue2

Interest rate3

Fish price4

Stand. Effort5

Cost per unit of effort6

Gill net 1 10.04 12.69 26.40 4.750 150.79 66.56Gill net 2 13.07 25.95 98.58 6.67 248.42 52.61Long line 8.49 25.51 200.47 9.80 144.49 58.75Scoop net 1 32.08 33.07 3.09 5.69 1688.05 19.00Shrimp basket trap 25.45 44.29 74.03 10.80 78.41 324.57Sprat scoop net 11.64 44.43 281.61 3.03 1402.48 8.30Trawl net 13.42 33.05 146.33 8.20 671.72 19.97Seines net 1 36.77 140.35 281.66 12.43 566.91 64.87Seines net 2 26.85 35.18 31.04 5.67 354.03 75.83Shrimp pull net 20.50 52.89 158.03 9.00 286.47 71.55Mussel trawl net 8.48 18.51 118.40 8.93 155.28 54.59Small cast net 7.17 15.60 117.57 5.00 167.86 42.71Lift net 2 10.05 11.93 18.65 5.50 259.13 38.80Big cast net 15.53 18.14 16.81 10.00 394.05 39.41Scoop net 2 24.23 27.31 12.70 5.50 368.13 65.82

1. Missing values of “Lift-net 1” and “Spears”. Data are calculated from data presented by 116 surveyed households in 2004 of FAU (2004) and 22 surveyed households in July 2005. 2. Data were estimated for operation of a fishing gear per year, with unit in million VNDs 3. Interest rate was estimated by Eq. (4.3), with unit in percent 4. Average harvested fish price (1,000 VNDs/kg) 5. Standardization effort was converted into standard units of Gillnet 1, with unit in number of days 6. Cost per unit of effort was calculated by Eq. (4.4), with unit in 1,000 VNDs/day of fishing.

∑

∑

=

== 17

1

17

1

7

7.

ii

iii

TCC

TCCPGAp (4.5)

- 34 -

4.3. Parameters estimation

When sustainable equilibrium occurs, CPUE is an index of stocked abundance that is

expressed in Eq. (3.8). It is that change in CPUE from one year to the next dependent

on effort and stocked fish levels. This is a complicated function whose parameters

a, q, r0 and K can be solved by a non-linear regression model (Gallant 1987). We

carried out the numerical solution of Eq. (3.8) by Mathematica 5.2 Software (Wolfram

2005). Data on Table 4.1, 4.3 and 4.4 were used for solving Eq. (3.8). The estimation of

parameters is showed in Table 4.6, Appendix 3, and the original CPUE equation is

expressed as:

Most parameters are statistically significant, with p-values less than 0.05. The R2

value is about 0.74, it means that 74% of CPUE variation is explained by the model.

Consequently, the relationship of CPUE with effort and stocking rates is significant

and has the expected signs. It indicates that stocking S is positively correlated to

changes in CPUE, whereas effort E is negatively correlated to changes in CPUE

values.

Table 4.6. Estimated parameters based on non-linear regression model

Parameters1: Estimate SE t-stat p-value CI q 0.093561 0.01808 5.17331 0.01402 0.036÷0.151K 0.20267 0.02539 7.97981 0.00411 0.122÷0.283r0 0.496084 0.01283 38.65784 0.00004 0.455÷0.537a 0.0117891 0.01437 0.82013 0.47223 -0.03÷0.058ANOVA table: DF SS MS R2 Variance

Model 4 0.000922 0.00023 0.73748 2.304x10-6

Error 3 6.91x10-6 2.31x10-6 Uncorrected Total 7 0.000929 Corrected Total 6 0.000026

1. CPUE(E,S) is catch per unit of effort (tons/day); q is catchability coefficient (tons/100,000

days/year); K is carrying capacity (100,000 tons); (r0 + aS) is intrinsic growth rate affected by

stocked fish; S is fish stocking (tons), and E is effort standardized (100,000 days). Assumption: there

is no change in fishing technology over time, biologically environmental conditions are constant,

and the multi-species fisheries is technical with no predator prey relationship.

+−=

SECPUE SE 0117891.0496084.0

018962.020267.0093561.0),( (4.6)

- 35 -

0

500

1,000

1,500

2,000

2,500

3,000

3,500

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

Act

ual c

atch

and

eff

ort .

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Act

ual C

PUE

.

Effort (number of households) Catch (tons) CPUE (kg/household)

Chapter V

V. RESULTS

5.1. Trends in catch, effort, CPUE and stocking

5.1.1. Trends of actual catch, effort and CPUE over time Catch data reflects aggregate freshwater fish instead of a single species of harvested

fish. This is because data on a particular species or even species groups are not

available for the study site. Catch and effort increased quickly from 1993 to 2000, and

then catch continued to increase and reached a peak of around 3,100 tons in 2002.

Whereas, the effort reduced slightly up to now. The corresponding CPUE fluctuated

largely over time, and has been declining in recent years (see Fig. 5.1).

Figure 5.1. Trends of actual catch, effort and CPUE over time

5.1.2. Trends of standardized effort, catch, CPUE and stocking over time

Trends of standardized effort, catch2 and stocking increased slightly from 2000 to

2003, and then dropped down after 2003 to 2005 (see Fig. 5.2 and Table 5.1).

Recently, the decreasing number of fishing days may be explained by fishermen

leaving or reducing their activity because of a poor fishing season, declining profit

and interruption of the stocking program. Figure 5.2 also depicts that the standard

- 36 -

0.00

2.00

4.00

6.00

8.00

10.00

12.00

1999 2000 2001 2002 2003 2004 2005

Stan

dard

ized

Effo

rt an

d St

ocki

ng

.

0

5

10

15

20

25

30

35

Stan

dard

ized

Cat

ch a

nd C

PUE

.

Standardized Effort (100,000 days) Stocking (tons)

Standardized CPUE (kg/day) Standardized Catch (100 kg)

CPUE decreased gradually between 2000 and 2003, after that CPUE reduced faintly

and tends to remain stable. This may indicate that fisheries resources were declining

initially, and may be a reason for fishermen leaving the fishery in recent years.

Figure 5.2. Trends in standardized catch, effort and CPUE, 1999-2005

Table 5.1. Standardized catch, effort and CPUE, 1999-2005

Year Standard effort

Fish stoking1

Standard CPUE

Standard catch2

Actual catch

Difference of catch3

(105days) (tons) (kg/day) (tons) (tons) (tons) (%) 1999 2.36 6.79 11.70 2,759 2,269 -490 -17.772000 1.70 8.94 13.94 2,373 2,301 -72 -3.022001 1.93 7.23 13.06 2,526 2,786 260 10.292002 2.46 11.06 11.99 2,952 3,118 166 5.632003 3.14 8.18 9.56 3,002 3,080 78 2.592004 2.90 5.69 9.82 2,850 2,835 -16 -0.552005 2.66 0 9.45 2,517 2,589 72 2.87

1. The fingerlings are often introduced into the reservoir at the end of year (August - December), so

the impact of stocking to population growth is assumed to begin from the following year. 2. “Standardized catch” or sustainable yield is theoretical harvest level, it is estimated by harvest

function (Eq. 3.7) of the modified surplus production model in Chapter III. 3. It presents the difference between the actual catch and the standardized catch, and the ratio of the

difference between the actual catch and the standardized catch to the standardized catch.

- 37 -

SECPUE SE 0117891.0496084.0

0017741.0018962.0),( +−= (5.1)

0

500

1,000

1,500

2,000

2,500

3,000

3,500

1999 2000 2001 2002 2003 2004 2005

Fish

pro

duct

ion

(tons

) ...

Actual catchStandar catch

Table 5.1 shows the ratio of the difference between actual catch and standardized

catch2 to the standardized catch fluctuated from 0.55% to 17.77%. Actual catch

tended to more than the standardized catch from 2001 (see Fig. 5.3); hence, the

fisheries resources have been diminished.

Figure 5.3. Trends of actual catch and standardized catch, 1999-2005

5.1.3. Relationship of standard CPUE with effort and stocking levels

Based on the parameters estimated by non-linear regression model (see Table 4.6),

and replacing these parameters into Eq. (3.8), the CPUE equation found is plotted in

Figure 5.4 and the below equation:

Where the unit of effort E is 100,000 days of fishing, stocking S is tons of stocked

fingerlings, and catch per unit of effort CPUE(E,S) is tons per day of fishing.

- 38 -

01

23

45 0

20

40

60

80100

00.0050.01

0.015

01

23

4 E

S

CPUE(E,S)

05000

1000015000

20000 0

20

40

60

80

100

02000400060008000

05000

1000015000

S

X

F(X,S)

Figure 5.4. Plotted curve of standardized CPUE vs. effort and stocking

5.2. Impact of stocking on population growth and harvest regime

5.2.1. Impact of stocking on population growth The MSPM found the population growth equation that was determined by

substituting the parameters estimation in Table 4.6 into Eq. (3.4), and expressed as:

Where the unit of stock biomass X is tons, stocking S is tons of stocked fingerlings,

and population growth F(X,S) is tons.

Figure 5.5. Population growth curve vs. stocking and stock biomass levels

( )

−+=

2026710117891.0496084.0),(

XXSF SX (5.2)

- 39 -

01

23

45 0

20

40

60

80

100

0

2000

4000

6000

01

23

4 E

S

H(E,S)

5.2.2. Relationship of the harvest regime with effort and stocking levels The harvest function for the capture fisheries based on Eq. (3.7), and by inserting of

parameters estimated from Table 4.6 is expressed as the Eq. (5.3) and is depicted in

Figure 5.6.


fingerlings, and harvest H(E,S) is tons.

Harvest function indicates that stocking is always positively trended to catch. Thus,

the fish stocking is positive impact to natural growth rate in population that is found

as the intrinsic growth rate equation:

Figure 5.6. Plotted curve of harvest vs. effort and stocking

5.3. Estimations of reference points and economics rents

5.3.1. Relationship of the fishing profit with effort and stocking levels The profit equation is aggregated from total revenue and total cost components; these

were found as the below equations and plotted in Figure 5.7.

i. Total cost of fishing activities includes cost of effort plus cost of stocking, as:

SEEH SE 0117891.0496084.0

41.1772.18962

),( +−= (5.3)

Srs 0117891.0496084.0 += (5.4)

SETC SE 257.6486),( += (5.5)

- 40 -

01

23

45

Effort

0

20

40

60

80

100

Stocking

-10000

-5000

0Profit

01

23

4EffortE

S

П(E,S)

ii. Total revenue of fishing activities is landed fish price multiplied by harvest, as:

iii. Profit of fishing activities equals total revenue minus to total cost, as:


fingerlings, and TR(E,S), TC(E,S), П(E,S) are million VNDs.

Figure 5.7. Plotted curve of profit vs. effort and stocking

5.3.2. Estimations of MSY, MEY, OAY and Economic rents When sustainable equilibrium occurs, the MSPM was used to calculate the indicators

of reference points for the fishery in the reservoir as presented in Table 5.2. The

parameterized harvest function (see Eq. 5.3) indicates that fish harvested was

positively correlated to the stocking rate. To estimate the reference points, the

stocking rate and effort level have to be solved simultaneously. The stocking rate

technically cannot exceed a given maximum stocking density, consequently, the

values of MSY, MEY, OAY and the corresponding effort levels are local maximum

values and limited in terms of a given range of stocking quantity.

SEETR SE 0117891.0496084.0

096.9737.104002

),( +−= (5.6)

SS

EESE 250117891.0496084.0

096.97396.39132

),( −+

−=∏ (5.7)

- 41 -

Table 5.2. Calculation indicators of the reference points and economic rents

Items: Harvest condition 11 Harvest condition 22 Harvest condition 33

MSY MEY OAY MSY MEY OAY MSY MEY OAY

Current status4

Effort5 (days of fishing) 265,113 99,767 199,534 315,514 118,734 232,243 895,133 336,855 602,258 266,444Catch (tons) 2,514 1,536 2,360 2,991 1,828 2,783 8,487 5,186 7,578 2,589Stocking6 (tons) 0 0 0 8.00 8.00 8.00 100 100 100 0Total Revenue (million VNDs) 13,787 8,424 12,943 16,408 10,025 15,265 46,550 28,443 41,567 14,200Total Cost7 (million VNDs) 17,197 6,472 12,943 20,666 7,902 15,265 60,565 24,351 41,567 19,197 + Cost for fishing (million VNDs) 17,197 6,472 12,943 20,466 7,702 15,065 58,065 21,851 39,067 19,197

+ Cost for stocking (million VNDs) 0 0 0 200 200 200 2,500 2,500 2,500 0

Profit (million VNDs) -3,410 1,952 0 -4,259 2,124 0 -14,015 4,092 0 -4,9971. Harvest condition 1: stocking quantity is limited or not introduced into the reservoir. 2. Harvest condition 2: stocking quantity is actually introduced into the reservoir, with an average of 48 fingerlings/ha (i.e. corresponding 1.2 million fingerlings/year or 8 tons fingerlings/year). This rate of stocking is suitable with investment capacity of the DNFC at present. 3. Harvest condition 3: stocking quantity is set up at optimal level. Stocking rate technically cannot exceed a given maximum stocking density, with about 600 fingerlings/ha (i.e. corresponding 15 million fingerlings/year or 100 tons fingerlings/year) (see Table 2.1). 4. The current fishing status (in 2005), data calculated from DNFC (2005). 5. Standardized effort measured in terms of number of fishing days, if it is converted to the corresponding number of households, it will be an average of 156 fishing days per household per year. 6. Weight of fish fingerlings was introduced into the reservoir at different levels of investment capacity. 7. Harvested fish price is 5,485 VNDs/kg; cost per unit of effort is 64,867 VNDs/days of fishing; cost per unit of fish stocking is 25,000 VNDs/kg of fingerlings; and an average of 150 fingerlings/kg.

- 42 -

Regarding the capture fisheries in Tri An reservoir, the harvest condition can be

separated to three levels that depend on given ranges of stocking quantity, as follows:

1. Harvest condition 1: fish stocking is limited or not introduced into the

reservoir.

2. Harvest condition 2: stocking quantity is still kept as the same as the

current level. Stocked density actually introduced was 48 fingerlings/ha.

This rate of stocking is suitable with the current investment capacity of the

DNFC.

3. Harvest condition 3: stocking quantity is set up at optimal stocking level.

Stocking rate technically cannot exceed a given maximum stocking

density, with about 600 fingerlings/ha.

As indicated in Table 5.2, the values of MSY level are different at the harvest

conditions. When these estimated values are compared with the actual catch and

effort values in Table 5.1, the MSY at the harvest condition 1 was attained back in

late 2001. At the harvest condition 2 the MSY level occurred back to 2002, whereas,

value of MSY at the harvest condition 3 was never reached before total catch stated

reducing (see Fig. 5.8).

On the other hand, there are different values of MEY level related to three harvest

conditions respectively. Comparing these values with the actual catch and effort

figures in Table 4.1 and Table 5.1, at the harvest condition 1 and 2, the MEY level

was attained back in late 1996 and 1997, respectively, while the MEY level at harvest

condition 3 has never been reached (see Fig. 5.8 and Fig. 5.9).

Regarding the issue of open access at different harvest conditions, Table 5.2 also

shows that the values of OAY level are different. Compared to figures in Table 4.1

and Table 5.1 indicated that values of the OAY level were reached in late 2000 and

2001 related to harvest conditions 1 and 2, respectively. At harvest condition 3, the

OAY level is still not attained, because the stocking program was not funded

sufficiently to ensure the maximum stocking quantity.

The computed total revenues, total costs and economic rents using the MSPM are also

shown in Table 5.2. Considering the economic efficiency, if operated at MSY target,

the fishing profit is always negative at all harvest conditions. Conversely, if operated

- 43 -

02.5

57.5

1099

99.2

99.4

99.6

99.8

100

02000400060008000

02.5

57.5

10

H(E,S)

E

S

•MSY

•MEY

01

23

45 99

99.2

99.4

99.6

99.8

100

-20000

2000

4000

01

23

4

Π(E,S)

E

S

•Πmax

at MEY target, the positive profit increases from 1,952 to 2,124 and 4,092 million

VNDs associated to the harvest condition 1, 2 and 3, respectively.

Figure 5.8. Harvest curve vs. stocking and effort at harvest condition 3

The graph is plotted from Eq. (5.3) in the case of optimal stocking; it shows that MSY may

reach at 8,487 tons when effort level is set up at 895,133 days of fishing. MEY value may

attain at 5,186 tons as long as the corresponding effort level is set up at 336,855 days.

Figure 5.9. Profit curve vs. stocking and effort at harvest condition 3

The graph is plotted from Eq. (5.7) in the case of optimal stocking; it shows that maximum

profit may attain at 4,092 million VNDs when effort level is set up at 336,855 days of

fishing.

- 44 -

Chapter VI

VI. DISCUSSION AND CONCLUSION

6.1. Discussion

6.1.1. Impact of fish stocking on population dynamics

Restocking and conservation measures are frequently implemented where the

fisheries resources have been overexploited or suffered environmental perturbation.

Introduction of fish into a reservoir may improve fish production where native stocks

have declined due to overfishing (Muli 1998; Welcomme 1998). Stocking may thus

be a method to meet to the problem of overexploitation (Cowx 2002). The major aims

of the stocking program in the reservoir are biological control and balancing a

depleted fish population, however the impact of stocking to fish population dynamics

was not considered in previous studies. This study initially points out the relationship

between stocking and the rate of population change. The MSPM describes one

possible interaction between fish stocking and population growth (see Eq. 5.2).

The stocking rate is found to be positively correlated to population growth, as the

change in population biomass per unit of time increased rapidly when many

fingerlings were introduced into the reservoir (see Fig. 5.5). Most of the stocked

species are non-predatory species, mostly plantivorus, zooplankton, organic detritus

and/or periphyton species, with the dominant species of Silver carp and Big head carp

(see Fig. 2.3). According to Luu (1998) and Luong et al. (2004), development of

these stocked species relies on natural food within the reservoir being ecologically

sound and most likely more sustainable. In addition, previous studies also point out

that these stocked fish are unable to reproduce locally or to adapt and compete

successfully with the local species in terms of growth and competition for food in the

reservoir (Ali 1998; An 2001; Hao 1997). Consequently, the stocking needs to

compensate for recruitment overfishing, and to maintain the fisheries productivity of

a water body at the highest possible level (Welcomme 1998). The findings of this

study and results of previous studies indicate that fish stocking in the reservoir is

positively related to changes in population growth and leads to a faster increase in the

population biomass compared with growth in the case of no stocking.

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6.1.2. Influence of fish stocking on harvesting regime In order to evaluate the role of stocking in enhancing fish production, the fish yield or

harvest level needs to be considered (An 2001; Cowx 1998). Stocking in inland

waters in Southeast Asia is proven and recognized as a successful methods of

increasing and sustaining fish catch (An 2001; Li & Xu 1995; Welcomme 1998). In

connection to this study, the findings in Figure 2.4 and Figure 5.2 showed that the

yield increased considerably when fish fingerlings were stocked into the reservoir,

compared to the year before the stocking program. The stocking was positively

trended to change in catch; it led to an increase in actual catch from 2,269 tons in

1999 to 3,118 tons in 2002, corresponding to an increase in stocking quantity. After

that, the actual catch reduced as a result of stocked fish quantity declining (see Table

5.1). The MSPM also found the harvest function for fishing activity in the reservoir

(see Eq. 5.3).

The change of harvest level depended on stocking rate and effort level (see Eq. 5.3

and Fig. 5.5). This indicates that the higher rate of stocking led to the greater yields of

harvested fish levels, while the effort rate was only positively increased in harvested

fish up to a certain limit, after which yields decline. Stocked fisheries can be managed

by changing the stocking rate or the fishing effort; however, they need to be

controlled by simultaneous change in both effort level and stocking rate. The

important issue of stocking management should define and note that high effort rates

combined with low stocking density leads to overfishing, conversely, leading to

overstocking/or superfluous stocking quantity (Lorenzen 1995). Welcomme (1998)

notes that effort and stocking are positively correlated and increases in yield should

be achieved by simultaneous increases in both effort and stocking rates, although an

initial reduction in effort may be needed to give the stocked population time to

establish.

6.1.3. Overfishing issues in the reservoir

In many cases, signs of overfishing are typically indicated in fisheries by decline in

CPUE, reduction in size of capture, loss of large-sized species, and change in

- 46 -

exploitation to smaller, less valued fish species (Clark 1985; Cowx 2002; Lokina

2000). In order to show whether or not overfishing indeed exists in the reservoir,

CPUE is an index of abundance and level of fisheries resources exploitation. The

MSPM indicated that relationship of CPUE with stocking and effort rates was defined

as Eq. (5.1).

The CPUE was in negative trend with effort level and positive trend with stocking

rate (see Eq. 5.1 and Fig. 5.4). The results in Table 5.1, Figure 5.2 and Figure 5.4

indicated that CPUE reduced gradually from 13.94 kg per fishing day in 2000 to 9.45

kg per fishing day in 2005, the tendency of CPUE corresponded to variations of effort

and stocking rates at that time. Regarding effort in number of fishing households (see

Fig. 5.1), the trend of CPUE decreased in the first three-year period, and then it

increased quickly until 2002 as the positive results of the stocking program became

evident. However, from 2003 up to now the CPUE has reduced steadily. It is

evidence to conclude that the current fisheries resources in the reservoir are

diminished and show signs of overfishing, and may suggest sustainability of the

resource is under threat. Tung et al. (2004) and Hao (1997) report that aquatic

resources of the reservoir have been reduced in terms of quantity and size of fish in

recent years.

The trend of catch also showed that actual fish harvested has been more than the

standardized catch from 2001 (see Table 5.1 and Fig. 5.3), so the fisheries resources

have tended to be degraded. In addition, the decline in effort in last three years may

be a sign of dwindling fisheries resources in the reservoir. Although the price of

harvested fish has increased recently as an impact of “Bird Flu Event” in late 2003,

the fishermen were still leaving the fishery. According to Lokina (2000), the

fishermen do not see the increase in fishing effort as a problem that was to be relating

to dwindling fish stocks. The interruption of the stocking program from 2004 was a

major reason leading to reduction in the number of fishing efforts. On the other hand,

the increase in the price of fuel may lead to decline of fishing profit, and this may be

another reason for many fishing households leaving the fishery or reducing their

fishing effort recently.

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6.1.4. Main factors accounting for overfishing

The trend of CPUE showed that the current fisheries resources in the reservoir are

overexploited. In order to gain insight into reasons leading to overfishing, several

factors need to be considered such as:

i. The dramatical increase in fishing effort level: although the entry to the

fishery was restricted to the local fishermen only, the fishing effort has

increased quickly from 300 households in 1993 to 1,470 households in 2002,

and then reduced gradually to 872 households in 2005 (see Table 4.1).

ii. Low awareness level of the fishermen about conservation issues: most

fishermen have a low education level (FAU 2000), it implies that fishing is

an easy livelihood for poor local people and that the provision of greater

livelihood opportunities within the existing fishery could thus lead to

increased fishing effort. Due to low education levels, most fishermen are still

limited in perception on conservation of fisheries resources. They continued

to use the prohibited fishing gears, and catch small fish with a higher

intensity of fishing.

iii. Ineffective measures of fisheries management: although fishing regulation

and controlling system for fishing activities were set up in 1995, the

implementation is limited and difficult to achieve expected targets due to

weakness of management capacity, and lack of knowledge and officers to

manage and monitor (Luu 1998; Tung et al. 2004). Many illegal fishermen

enter the fishery without fishing license/without control as the result of a

lack of officers in license control and monitoring (DNFC 2005). The fishery

has often been mismanaged, usually in terms of regulating catch and access,

consequently the fish stock are overexploited or unable to support the

fishing pressure imposed (Cowx 2002).

iv. Inappropriate strategies of the stocking program: the stocking program

attracted a larger number of new fishermen to the fishery, with 748

households in 1996 and reached a peak at 1,470 households in 2002

compared with 400 in 1994 before the stocking program (see Table 4.1).

However, the stocking strategies were inappropriate, fish fingerlings were

stocked into the reservoir at low density (48 fingerlings/ha) compared with

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regulation of stocking (see Table 2.1 and Fig 2.4). Thus, stocking could not

provide a sufficient quantity of catch demands to increasingly higher fishing

pressure levels. Lorenzen (1995) points out that low stocking density

combined with high fishing effort level lead to overfishing.

v. The use of illegal fishing gears and violation of fishing regulations: gear

limitations and fishing regulation were also set up in 1995; however, many

fishermen did not follow the regulations. Many violators of fishing

regulations still operated these types of prohibited gear that result in

destructive fishing activities such as explosives, toxicants, and certain other

destructive fishing methods (eg. filtering barrier with small mesh size). For

example, in 2005, the DNFC detected 165 violators who either used gears

with small mesh size, explosives and/or operated in closed areas.

6.1.5. Sustainability and efficiency issues In biological terms, MSY is a reference point used for control of the harvest regime.

The MSY is itself based on a biological model and indicates that a fish stock cannot

be exploited too heavily and shows the maximum harvest level that ensures

sustainable development (Clark 1990). To assess whether or not ecological

sustainability of the current stocking and fishing activities in the reservoir indeed

exist, the MSY level was estimated and compared with actual catches.

Theoretically, if the actual catch, effort level and stocking rate exceed the values of

the MSY level, it indicates that the current fisheries exploitation is unsustainable. The

findings report that at the harvest condition 1 and 2, sustainable yields increased and

reached values of the MSY level in late 2001 and 2002 respectively, and then

reduced below MSY level from 2004 (see Table 5.1 and Table 5.2). Therefore, the

fishery moved into a situation of biological overfishing. Currently, the interruption of

stocking time (i.e. harvest condition 1), actual effort level was more than the EMSY

from 2003, while actual stocking rate was too small compared with required SMSY.

This indicates that the current fisheries moved into a situation of biological

overfishing, because the high effort level combined with low stocking density lead to

overfishing (Lorenzen 1995). The MSY were also less than the actual catch from

2002, which is evidence that the current fishery was unsustainable. However,

- 49 -

regarding the harvest condition 3 values of MSY level have not yet been reached, this

MSY level only reaches at 8,487 tons when ensured by simultaneously set up EMSY at

895,133 fishing days and SMSY at 100 tons of fish fingerlings (see Table 5.2 and Fig.

5.8). This MSY target may be impossible/or ineffective to be applied to management

in the reservoir, because it is very difficult to implement in terms of monitoring and

controlling, especially for a large reservoir as Tri An.

Regarding economics, economic efficiency occurs when the sustainable catch or

effort level and stocking rate for the fishery as a whole maximizes profits. This point

is referred as MEY that is the largest difference between total revenue and total cost

of fishing and stocking (Clark 1985). At the MEY level, the stock biomass is larger

than that associated with MSY level, so the economic objective of MEY is better than

that of MSY in protecting the fishery from negative environmental shocks that may

diminish the fish population (Kompas 2005). Catch and effort levels at MEY will

vary due to a change in the price of harvested fish or the cost of fishing and stocking.

As long as the cost of fishing and stocking increase, the MEY as a target will always

be preferred to MSY, the harvested fish at the MSY level becomes economic

overfishing (Kompas 2005).

The findings show that MEY at the harvest condition 1 and 2 attained at 1,536 and

1,828 tons when the corresponding EMEY was produced at 99,767 and 118,734 days of

fishing, respectively (see Table 5.2). Comparing these values at MEY with the actual

data, the fishery sustained economic overfishing from 1996 up to now. The main

reasons of economic overfishing were inappropriate stocking strategies and poor

management. Regarding the harvest condition 3, the MEY can be reached at 5,186

tons, and produced at effort of 336,855 fishing days (see Table 5.2 and Fig. 5.8),

however, this MEY level was never attained before. This MEY target may be applied

for reservoir management, because it may be suitable with the existing management

capacity.

6.1.6. Required rational management of stocked fisheries

The findings from the MSPM indicate that the current fisheries exploitation level

constitutes overfishing and is unsustainable. At harvest condition 1 and 2, the current

- 50 -

effort level exceeded the sustained effort at MSY, MEY and OAY. The fish

fingerlings will continue to be introduced into the reservoir, thus a rational

management of stocked fisheries is required in order to guarantee sustainability and

efficiency of fish resources exploitation that can be exploited year after year

indefinitely into the future. The optimal stocking regime is dependent on the

harvesting regime and vice versa. Fishing activities are dependent on the harvesting

and stocking regimes, thus, a change in the fisheries management plan should be

ensured by simultaneous change in both stocking rate and effort level, and need to

avoid overstocking or overfishing (Lorenzen 1995). The following management

guidelines should be considered in order to improve the current fisheries

management:

i. If the fishery operation is maintained at harvest condition 2 in which 8 tons

of fingerlings are annually introduced into the reservoir, then change in

fishing effort levels and stocking rates to achieve levels at the reference

points may recover the current fisheries resources from degradation. In this

context, the current effort should increase by 18.42% and 8 tons of fish

fingerlings would be introduced into the reservoir to get the corresponding

MSY level. Whereas, to attain the MEY and OAY the current effort levels

have to be decreased by 55.44% and 12.84% respectively, and the stocking

rates sets up at 8 tons of stocked fingerlings.

ii. Considering harvest condition 3, the MSY and OAY targets seem to be

exceeding the management capacity, or are impossible and ineffective to be

applied in the large reservoir. Only MEY target may be possible to be

implemented for reservoir management. The current effort level is still less

than the effort level at MEY, hence, in order to achieve this MEY target the

current effort level would need to be increased by 26.43% while the

stocking rate would need to be 100 tons of stocked fingerlings.

6.1.7. Limitations of the study and model Data used in this study were collected mostly from the DNFC, so the quality of an

assessment of capture fishery status and management was very dependent on the

input information that can be limited in accuracy and reliability due to poor

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management and poor stocking strategies. The R2 value of the non-linear regression

model was limited due to a short time series of data. Based on 138 surveyed fishing

households in 2004 and 2005, cost per unit of effort was calculated and assumedly

used for the reference points and economic rents estimation in this study, therefore

the results were still limited in accuracy.

The MSPM was assumed and has been developed for stock assessment in the

reservoir; however, it has disadvantages that should be improved in future studies.

The MSPM used and is based on some assumptions, and if these assumptions are not

met such as change of catchability or environmental factors, then the MSPM is not

suitable. The MSPM only used the catch/effort and stocking data to estimate the

biological parameters by non-linear regression, so the findings were limited in

accuracy because of the multi-species nature of the fisheries resource and lack of long

historical data series. In addition, the bioeconomics approach including both biology

of the stock and economics of the fishery was also modified based on the MSPM.

This approach required many parameters concerning all aspects of fishing activities

such as costs of effort/stocking, prices, etc., therefore, the complexity of the model

increases its uncertainties. Moreover, the high fluctuation of landed fish price and

cost of fishing effort were also reasons leading to uncertainties of this approach.

Based on the limitations addressed, in future the MSPM should be applied for single

species that are commercial species. The impact of fish stocking will be tested more

accurately based on the biological relationship between stocked species and native

species. To ensure greater certainty of the MSPM and bioeconomics approach, future

studies should use long data time-series of catch, effort and stocking, and more

detailed and accurate fish price and cost of fishing by month.

6.2. Conclusion and management implications

The surplus production model of Verhulst-Schaefer was modified and it initially

pointed out the relationship between fish stocking and the rate of population change.

The model assumed that fish stocking in the reservoir was positively correlated to

population growth and led to rapid increase in population growth compared with

change in the case of no stocking impact, which was confirmed by available data. To

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evaluate the role of stocking in enhancement of fish production, the findings also

found that harvest level was positively related to change of stocking rate, and the

higher rate of stocking led to the greater yields of harvested fish. The harvest also

depended on the effort level, thus stocked fisheries can be managed by simultaneous

change in both effort level and stocking rate.

Empirically analyze and evaluate the status of fisheries resources exploitation affected

by the stocking program were implemented. Empirical results indicated that CPUE

declined steadily in recent years; therefore the current fisheries resources were

overexploited and have been degraded. There were many factors leading to

overexploitation of the fisheries resources, however, major factors can be specified as

dramatic increase in fishing effort, low awareness level of fishermen of conservation

issues, ineffective methods of management, and the use of illegal fishing gears and

violation of fishing regulations. In addition, the stocking strategies were

inappropriate, fish stocking of low density combined with high fishing effort also led

to overfishing.

The current exploitation of fisheries resources in the reservoir has moved into an

unsustainable situation of biological and economic overfishing. The main reason was

an excess of the current effort combined with low stocking rate compared with the

corresponding levels associated with the MSY target in harvest conditions 1 and 2.

On the other hand, the actual catch, effort and stocking rates also exceeded the levels

associated with the MEY target in these cases; hence, the fishery sustained economic

overfishing from 1996 up to the present. The current centralized top-down

management has proven ineffective and has faced many problems in fisheries

management, hence, rational and improved management are required in order to

ensure sustainability and efficiency of fish resources exploitation that can be

obtainable each year indefinitely into the future.

Sustainability and efficiency issues in a fishery, or pursuing MSY and MEY targets,

are important. To manage fisheries resources, the MSY and MEY are the simplest

reference points used to control the harvest regime and protect the fish population.

The values of MSY and MEY estimations based on the MSPM also helped to

evaluate biological sustainability and economic efficiency. The MSPM indicates that:

- 53 -

i. Regarding the biological side, if the fishery operates at harvest condition 3,

MSY reaches at 8,487 tons, with the effort level of 895,133 days. At present,

it would not be possible or effective to apply this MSY target to the Tri An

reservoir.

ii. From an economic point of view, if the fishery operates at harvest condition 3,

the MEY attains at 5,186 tons as long as the corresponding effort level is set

up at 336,855 days of fishing. This MEY target may be possible and can be

applied for fisheries resources exploitation because the current effort level is

less than the effort at MEY level and is suited to current management

capacity.

Fisheries management in Tri An reservoir is based on a centrally controlled, top–

down approach under provincial patronage, with the local authorities acting as the

centralized management authority. There are still many problems in fisheries

management related to the reservoir, for example, the current legal framework for

management was not yet suitable, protection of the fisheries resources was still very

limited, and funds for the stocking program into the reservoir was low. The current

fisheries management regime should be improved. This study provided preliminary

results on the status of stock and fisheries management to build and recommend

several possible measures for reservoir fisheries management.

The findings indicate that current fisheries resources are overfished and fishing

activities unsustainable. Since the fisheries resources have been depleted by

overfishing, the following management plans should be considered (Lorenzen et al.

2001), as follows:

1) Stocking to develop a largely culture-based fishery while maintaining high

exploitation rates; or

2) Supplemental stocking combined with more restricted harvesting to rebuild

natural spawning stocks more quickly than would be possible through harvest

restrictions alone.

Fisheries policy may be a choice between option (1) and option (2). In order to

improve the current fisheries management for the reservoir, option (1) seems to be the

- 54 -

MSY target at harvest condition 3. This option is not feasible when applied to a large

reservoir as Tri An, because the stocked fish have a low survival rate in a large

reservoir and it is difficult to appropriately manage. Whereas option (2) seems to be

the MEY target at harvest condition 3, it may be possible for the reservoir fisheries

management because of the stocking program to compensate for recruitment

overfishing and to maintain the fisheries productivity at the possible harvest level.

Moreover, the MEY target is more suitable with existing management capacity.

However, in implementation of new management measures the following need to be

considered (Lokina 2000):

i. The policy should be flexible enough to allow for proper reaction to changes

in economic and biological conditions; and

ii. Involve the participation and support of the local communities and ensure

minimum resistance.

To implement simultaneously in both the restricted harvesting and supplemental

stocking, there are some effective measures for the control of fishing and protection

of fish resources (Bhukaswan 1980; Clark 1985; Cowx 2002), as follows:

1. Time and place restriction measures are more suitable for the reservoir

because the species caught varies from area to area, or from season to season

within the same area (DNFC 1997).

2. Regulation of the minimum size of fish caught, particularly by controlling the

mesh size of the fishing gear is difficult to enforce in the reservoir due to the

multi-species resources and multi-gears used. The application of this measure

must be considered carefully in the reservoir because it is impossible to set a

proper size limit suited for all species. However, regulation of mesh size

should be used for controlling the minimum commercial size in protected fish

populations in order to protect immature fish and first spawning fish.

3. Limitation of entry by limiting the fishermen and/or fishing units. This

management measure is also difficult to implement in this fishery since the

reservoir fisheries are open access and important firstly as a source of protein

and secondly as providers of employment. Moreover, one of purposes of the

fishery is to supply employment for people living around the reservoir, any

regulation that limited employment, either directly or indirectly, would not be

- 55 -

favorable (Amarasinghe & De Silva 1999; Kapetsky 1983). However, to get

the MEY target at harvest condition 3, this measure is not a serious problem

due to the current effort level less than EMEY. In this case, the policy makers

should choose an optimal stocking regime that have in order to avoid the

overfishing/overstocking situations.

4. The existing illegal fishing gears, non-registered fishermen and violation of

fishing regulations are the biggest social issues in the reservoir. Therefore, in

order to successfully implement the prohibition of destructive fishing

activities by prohibition of the use of explosives, toxicants, and certain other

destructive fishing methods, the policy actions on education of people living

around the reservoir need to be considered foremost. Training courses should

focus on protection of fisheries resources and the benefits of managed

fisheries resources to promote awareness of the fishing communities about

fisheries resource protection.

The current management regime through a centralized top-down management

approach has also proven ineffective in addressing and resolving the above issues,

primarily because of the absence of participation with the local communities. In the

search for alternative reservoir management approaches, the most promising option

appears to be co-management (Lorenzen et al. 2001; Nathanael & Edirisinghe 2002),

where all stakeholders, especially fishermen, collaborate with the government, to seek

solutions to fishery related issues. The essential idea of co-management is the sharing

of decision making and management functions between government and stakeholders

in the fishery (Bavinck et al. 2005; Charles 2001). Moreover, an important advantage

of a co-management approach for managers is that through co-operative societies,

more reliable catch and effort statistics can be collected which are useful not only for

monitoring the status of the fisheries but also for planning welfare programs

(Amarasinghe & De Silva 1999).

In the reservoir, co-management tasks such as the introduction of a licensing system

for fishing, setting up a monitoring system, establishment of a revolving fund to

ensure continuous supplies of fingerlings and promotion of a managed hatchery

should be implemented as a necessity. The fishing communities who invest funds in

stocking activities through fishing taxes should be interested in fisheries management.

- 56 -

The participation of these communities in fisheries governance may ensure that

stocked fish are harvested at optimum yield and fishermen who do not participate

financially in stocking activities are kept out. Furthermore, the co-management

approach allows all stakeholders to take roles in the governance process and to have

the chance for sharing of responsibility and power. The work pressure of the DNFC

and provincial government may be reduced, because other stakeholders as fishermen,

farmed-fishers and wholesalers will be directly involved in governance to monitor and

control all fishing activities.

- 57 -

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APPENDICES

Appendix 1. Data on fish stocking in Tri An reservoir, 1995-2003 [Source: data cited from DNFC (2005)]

STATISTICAL DATA IN 1995 STATISTICAL DATA IN 1996 STATISTICAL DATA IN 1997 Species name

Scientific name

Biological features Weight1 Quantity2 Cost3 Weight1 Quantity2 Cost3 Weight1 Quantity2 Cost3

Silver carp H.molitris Planktivorous 2,105 314,317 52.63 3,077 459,387 76.92 8,106 1,210,363 202.66 Bighead carp A.nobilis Planktivorous 1,102 160,822 27.54 1,610 235,048 40.25 4,242 619,290 106.06 Grass carp C.idellus Herbivorous 793 114,320 19.82 1,159 167,083 28.97 3,054 440,221 76.34 Nile tilapia O.niloticus Detritivorous 1,026 150,120 25.64 1,499 219,406 37.48 3,950 578,078 98.74 Common carp C.carpio Detritivorous 1,049 162,310 26.22 1,533 237,222 38.33 4,039 625,018 100.98 Indian carp L.rohita Detritivorous 887 120,759 22.16 1,296 176,494 32.39 3,414 465,016 85.35 Mud carp C.molitorella Detritivorous 1,110 163,925 27.74 1,622 239,583 40.54 4,273 631,239 106.82 Java barb B.gonionotus Detritivorous 755 113,426 18.88 1,104 165,776 27.59 2,908 436,776 72.69

1. Weight of stocked fish, with unit in kg

2. Quantity of stocked fish, with unit in number of fingerlings

3. Total cost of stocked fish, with unit in million VNDs

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Appendix 1. Data on fish stocking in Tri An reservoir, 1995-2003 (cont.) [Source: data cited from DNFC (2005)]


Scientific name


Silver carp H.molitris Planktivorous 1,619 241,782 40.48 2,133 318,427 53.32 1,630 268,425 40.75 Bighead carp A.nobilis Planktivorous 847 123,709 21.19 1,116 162,925 27.90 0 0 0.00 Grass carp C.idellus Herbivorous 610 87,939 15.25 803 115,815 20.08 970 178,550 24.25 Nile tilapia O.niloticus Detritivorous 789 115,477 19.72 1,039 152,083 25.98 1,120 176,115 28.00 Common carp C.carpio Detritivorous 807 124,854 20.17 1,063 164,433 26.57 1,225 212,350 30.63 Indian carp L.rohita Detritivorous 682 92,892 17.05 898 122,338 22.45 0 0 0.00 Mud carp C.molitorella Detritivorous 854 126,096 21.34 1,124 166,069 28.10 1,518 237,863 37.95 Java barb B.gonionotus Detritivorous 581 87,251 14.52 765 114,909 19.12 770 127,330 19.25


Scientific name


Silver carp H.molitris Planktivorous 2,262 326,300 56.55 2,418 345,774 60.45 1,362 205,030 34.05 Bighead carp A.nobilis Planktivorous 1,216 172,505 30.40 1,776 255,765 44.40 1,023 157,847 25.58 Grass carp C.idellus Herbivorous 1,087 116,944 27.18 359 50,978 8.98 474 70,168 11.85 Nile tilapia O.niloticus Detritivorous 1,278 173,763 31.95 868 126,728 21.70 472 70,507 11.80 Common carp C.carpio Detritivorous 940 132,456 23.50 832 117,416 20.80 826 129,317 20.65 Indian carp L.rohita Detritivorous 2,374 315,763 59.35 531 75,296 13.28 326 49,048 8.15 Mud carp C.molitorella Detritivorous 1,157 158,942 28.93 612 86,904 15.30 757 113,717 18.93 Java barb B.gonionotus Detritivorous 742 103,492 18.55 782 109,844 19.55 458 72,714 11.45

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Appendix 2. Data on capture fisheries activities in Tri An reservoir, 1999-2005 [Source: data cited from DNFC (2005)]

STATISTICAL DATA IN 1999 STATISTICAL DATA IN 2000 Type of fishing gear No.of

gears1 No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

No.of gears1

No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

Sprat scoop net 134 134 134 16,120 724,240 2 68 68 68 6,808 652,120 2Scoop net 1 40 40 19 2,850 124,050 5 52 52 52 2,530 225,000 5Scoop net 2 25 25 25 2,750 72,500 4 2 2 2 210 4,840 4Shrimp basket trap 13,050 279 156 23,200 109,240 7 21,360 356 356 46,140 195,750 7Trawl net 46 46 46 9,350 120,450 4 38 38 38 3,120 44,920 4Gill net 1 452 452 452 77,125 742,050 4 526 526 526 56,350 761,850 4Gill net 2 48 48 48 4,560 156,050 4.5 114 114 114 9,250 80,980 4.5Mussel trawl net 8 8 8 610 7,320 5 3 3 3 945 9,209 5Lift net 1 6 6 6 450 3,650 4.5 17 17 17 812 8,050 4.5Lift net 2 38 38 38 2,650 119,700 3 22 22 22 920 15,450 3Long line 25 25 25 3,750 16,520 7 132 132 132 9,140 69,560 7Spears 9 9 9 567 1,710 10 1 1 1 93 274 10Small cast net 12 12 12 1,720 20,790 5 27 27 27 2,020 32,000 5Big cast net 0 0 0 0 0 0 38 38 38 1,550 66,350 5Seines net 1 0 0 0 0 0 0 35 35 35 1,550 75,790 4Seines net 2 14 14 14 1,900 50,400 4 39 39 39 1,925 59,120 4Shrimp pull net 0 0 0 0 0 0 0 0 0 0 0 0 1. Number of gears, 2. Number of households, 3. Number of boats, 4. Number of fishing days, 5. Catch with unit in kg, 6. Price with unit in 1,000 VNDs/kg

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Appendix 2. Data on capture fisheries activities in Tri An reservoir, 1999-2005 (cont.) [Source: data cited from DNFC (2005)]


gears1 No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

No.of gears1

No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

Sprat scoop net 78 78 78 7,850 844,606 2 58 58 58 9,900 994,776 2Scoop net 1 68 68 68 7,600 670,600 5 42 42 42 6,800 654,756 5Scoop net 2 3 3 3 596 34,500 4 1 1 1 100 3,897 4Shrimp basket trap 26,320 378 214 27,250 126,240 7 25,500 304 234 52,850 249,450 7Trawl net 68 68 68 6,020 82,320 6 28 28 28 7,600 109,662 6Gill net 1 197 197 197 32,325 465,625 4 157 157 157 31,250 395,768 4Gill net 2 90 90 90 18,550 145,350 6 90 90 90 22,950 187,560 6Mussel trawl net 14 14 14 950 9,350 7 14 14 14 1,850 21,560 7Lift net 1 41 41 41 1,450 17,250 5 22 22 22 1,950 23,552 5Lift net 2 52 52 52 2,160 53,790 4 36 36 36 2,780 59,977 4Long line 111 111 58 8,710 87,025 7 42 42 42 8,160 98,976 7Spears 2 2 2 189 850 12 0 0 0 0 0 0Small cast net 32 32 32 3,420 52,430 5 26 26 26 3,900 58,950 5Big cast net 19 19 19 845 31,420 8 10 10 10 1,500 49,242 9Seines net 1 17 17 17 850 40,250 8 10 10 10 1,050 47,545 9Seines net 2 28 28 28 1,950 59,200 4 21 21 21 2,700 72,685 4Shrimp pull net 39 39 39 5,125 64,725 7 31 31 31 6,725 89,505 7 1. Number of gears, 2. Number of households, 3. Number of boats, 4. Number of fishing days, 5. Catch with unit in kg, 6. Price with unit in 1,000 VNDs/kg

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Appendix 2. Data on capture fisheries activities in Tri An reservoir, 1999-2005 (cont.)

[Source: data cited from DNFC (2005)] STATISTICAL DATA IN 2003 STATISTICAL DATA IN 2004

Type of fishing gear No.of

gears1 No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

No.of gears1

No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

Sprat scoop net 61 61 61 8,925 884,776 2.5 60 60 60 8,288 809,888 4Scoop net 1 42 42 42 7,902 694,756 5.5 40 40 40 7,501 661,628 9Scoop net 2 1 1 1 54 3,697 5.5 1 1 1 27 1,849 5.5Shrimp basket trap 27,300 273 273 48,250 249,450 9 25,300 253 253 43,625 207,175 18Trawl net 26 26 26 3,248 109,662 7 22 22 22 2,686 115,081 10Gill net 1 267 267 267 40,575 397,768 4.5 252 252 252 38,038 371,384 6Gill net 2 88 88 88 21,750 197,560 6 76 76 76 18,250 196,030 10Mussel trawl net 14 14 14 2,150 21,560 8 14 14 14 2,455 21,280 15Lift net 1 22 22 22 2,850 23,552 5.5 20 20 20 2,850 23,026 8Lift net 2 31 31 31 2,880 59,977 5.5 31 31 31 3,215 67,239 5.5Long line 39 39 39 13,250 98,976 8 39 39 39 11,279 93,738 20Spears 0 0 0 0 0 0 0 0 0 0 0 0Small cast net 36 36 36 4,150 58,950 5 34 34 34 3,925 55,725 6Big cast net 18 18 18 2,920 69,242 10 14 14 14 2,050 53,863 16Seines net 1 10 10 10 1,250 47,545 12 7 7 7 1,335 45,273 15Seines net 2 19 19 19 2,350 72,685 5 19 19 19 1,989 66,593 7Shrimp pull net 31 31 31 2,850 89,505 9 16 16 16 1,425 44,753 9 1. Number of gears, 2. Number of households, 3. Number of boats, 4. Number of fishing days, 5. Catch with unit in kg, 6. Price with unit in 1,000 VNDs/kg

- 67 -

Appendix 2. Data on capture fisheries activities in Tri An reservoir, 1999-2005 (cont.) [Source: data cited from DNFC (2005)]


gears1 No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

No.of gears1

No.of hhs2

No.of boats3

No.of f.days4

Total catch5

Fish price6

Sprat scoop net 58 58 58 7,650 735,000 4 - - - - - -Scoop net 1 38 38 38 7,100 628,500 9 - - - - - -Scoop net 2 0 0 0 0 0 0 - - - - - -Shrimp basket trap 25,090 295 232 39,000 164,900 18 - - - - - -Trawl net 17 17 17 2,124 120,500 10 - - - - - -Gill net 1 236 236 236 35,500 345,000 6 - - - - - -Gill net 2 64 64 64 14,750 194,500 10 - - - - - -Mussel trawl net 14 14 14 2,760 21,000 15 - - - - - -Lift net 1 17 17 17 2,850 22,500 8 - - - - - -Lift net 2 31 31 31 3,550 74,500 5.5 - - - - - -Long line 39 39 39 9,308 88,500 20 - - - - - -Spears 0 0 0 0 0 0 - - - - - -Small cast net 31 31 31 3,700 52,500 6 - - - - - -Big cast net 9 9 9 1,180 38,484 16 - - - - - -Seines net 1 4 4 4 1,420 43,000 15 - - - - - -Seines net 2 19 19 19 1,628 60,500 7 - - - - - -Shrimp pull net 0 0 0 0 0 0 - - - - - - 1. Number of gears, 2. Number of households, 3. Number of boats, 4. Number of fishing days, 5. Catch with unit in kg, 6. Price with unit in 1,000 VNDs/kg

- 68 -

Appendix 3. Results of non-linear regression statistics of CPUE function

The software used is “Mathematica 5.2.0 Windows” (Wolfram 2005)

Where: CPUE is catch/effort (tons/day of fishing); e is effort

standardization (100,000 days of fishing); sw1 is stocking (tons of

stocked fingerlings); q is catchability coefficient (tons/100,000

fishing days/year); k is carrying capacity (100,000 tons); and (r0+

a*sw1) is intrinsic growth rate which affected by stocking.

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Appendix 4. Photographs of main fishing gears used

[Source: photos cited from AMCF (2002)]

Source of photographs in the front page:

(1) Photos showing main fish species introduced into the Tri An reservoir cited

from MRC (2003).

(2) Photo showing Tri An Hydro-Electric Plant cited from

http://www.dongnai-industry.gov.vn/english/tp_bh.html#vc

(3) Photo showing “Scoop net” cited from AMCF (2002)

Shrimp basket trap Gill-net

Sprat scoop net Long line

lam pt status of capture fisheries activities and mangemen

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